Cambridge Natural Science Manuals.
Biological Series.

General Editor:—Arthur E. Shipley, M.A.
FELLOW AND TUTOR OF CHRIST’S COLLEGE, CAMBRIDGE.

FOSSIL PLANTS.

London: C. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE,
AND
H. K. LEWIS,
136, GOWER STREET, W.C.

Glasgow: 263, ARGYLE STREET.
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay: E. SEYMOUR HALE.

Tree Stumps in a Carboniferous Forest. Victoria Park, Glasgow.

FOSSIL PLANTS

FOR STUDENTS OF BOTANY AND GEOLOGY

BY

A. C. SEWARD, M.A., F.G.S.
ST JOHN’S COLLEGE, CAMBRIDGE,
LECTURER IN BOTANY IN THE UNIVERSITY OF CAMBRIDGE.

WITH ILLUSTRATIONS.

VOL. I.

CAMBRIDGE:
AT THE UNIVERSITY PRESS.
1898

[All Rights reserved.]

Cambridge:
PRINTED BY J. AND C. F. CLAY,
AT THE UNIVERSITY PRESS.

PREFACE.

IN acceding to Mr Shipley’s request to write a book on Fossil Plants for the Cambridge Natural History Series, I am well aware that I have undertaken a work which was considered too serious a task by one who has been called a “founder of modern Palaeobotany.” I owe more than I am able to express to the friendship and guidance of the late Professor Williamson; and that I have attempted a work to which he consistently refused to commit himself, requires a word of explanation. My excuse must be that I have endeavoured to write a book which may render more accessible to students some of the important facts of Palaeobotany, and suggest lines of investigation in a subject which Williamson had so thoroughly at heart.

The subject of Palaeobotany does not readily lend itself to adequate treatment in a work intended for both geological and botanical students. The Botanist and Geologist are not always acquainted with each other’s subject in a sufficient degree to appreciate the significance of Palaeobotany in its several points of contact with Geology and recent Botany. I have endeavoured to bear in mind the possibility that the following pages may be read by both non-geological and non-botanical students. It needs but a slight acquaintance with Geology for a Botanist to estimate the value of the most important applications of Palaeobotany; on the other hand, the bearing of fossil plants on the problems of phylogeny and descent cannot be adequately understood without a fairly intimate knowledge of recent Botany.

The student of elementary geology is not as a rule required to concern himself with vegetable palaeontology, beyond a general acquaintance with such facts as are to be found in geological text-books. The advanced student will necessarily find in these pages much with which he is already familiar; but this is to some extent unavoidable in a book which is written with the dual object of appealing to Botanists and Geologists. While considering those who may wish to extend their botanical or geological knowledge by an acquaintance with Palaeobotany, my aim has been to keep in view the requirements of the student who may be induced to approach the subject from the standpoint of an original investigator. As a possible assistance to those undertaking research in this promising field of work, I have given more references than may seem appropriate to an introductory treatise, and there are certain questions dealt with in greater detail than an elementary treatment of the subject requires. In several instances references are given in the text or in footnotes to specimens of Coal-Measure plants in the Williamson cabinet of microscopic sections. Now that this invaluable collection of slides has been acquired by the Trustees of the British Museum, the student of Palaeobotany has the opportunity of investigating for himself the histology of Palaeozoic plants.

My plan has been to deal in some detail with certain selected types, and to refer briefly to such others as should be studied by anyone desirous of pursuing the subject more thoroughly, rather than to cover a wide range or to attempt to make the list of types complete. Of late years there has been a much wider interest evinced by Botanists in the study of fossil plants, and this is in great measure due to the valuable and able work of Graf zu Solms-Laubach. His Einleitung in die Palaeophytologie must long remain a constant book of reference for those engaged in palaeobotanical work. While referring to authors who have advanced the study of petrified plants of the Coal period, one should not forget the valuable services that have been rendered by such men as Butterworth, Binns, Wilde, Earnshaw, Spencer, Nield, Lomax and Hemingway, by whose skill the specimens described by Williamson and others were first obtained and prepared for microscopical examination.

I am indebted to many friends, both British and Continental, for help of various kinds. I would in the first place express my thanks to Professor T. McKenny Hughes for having originally persuaded me to begin the study of recent and fossil plants. I am indebted to Prof. Nathorst of Stockholm, Dr Hartz of Copenhagen, Prof. Zeiller, Dr Renault and Prof. Munier-Chalmas of Paris, Prof. Bertrand of Lille, Prof. Stenzel and the late Prof. Roemer of Breslau, Dr Sterzel of Chemnitz, the late Prof. Weiss of Berlin, the late Dr Stur of Vienna, and other continental workers, as well as to Mr Knowlton of Washington, for facilities afforded me in the examination of fossil plant collections. My thanks are due to the members of the Geological and Botanical departments of the British Museum; also to Mr E. T. Newton of the Geological Survey, and to those in charge of various provincial museums, for their never-failing kindness in offering me every assistance in the investigation of fossil plants under their charge. Prof. Marshall Ward has given me the benefit of his criticism on the section dealing with Fungi; and my friend Mr Alfred Harker has rendered me a similar service as regards the chapter on Geological History. I am especially grateful to my colleague, Mr Francis Darwin, for having read through the whole of the proofs of this volume. To Mr Shipley, as Editor, I am under a debt of obligation for suggestions and help in various forms. I would also express my sense of the unfailing courtesy and skill of the staff of the University Press.

My friend Mr Kidston of Stirling has always generously responded to my requests for the loan of specimens from his private collection. Prof. Bayley Balfour of Edinburgh, Mr Wethered of Cheltenham and others have assisted me in a similar manner. I would also express my gratitude to Dr Hoyle of Manchester, Mr Platnauer of York, and Mr Rowntree of Scarborough for the loan of specimens.

To Dr Henry Woodward of the British Museum I am indebted for the loan of the woodblocks made use of in figs. 10, 47, 60, 66, and 101, and to Messrs Macmillan for the process-block of fig. 25.

For the photographs reproduced in figs. 15, 34, 68, 102 and 103 I owe an acknowledgment to Mr Edwin Wilson of Cambridge, and to my friend Mr C. A. Barber for the micro-photograph made use of in fig. 40.

In conclusion I wish more particularly to thank my wife, who has drawn by far the greater number of the illustrations, and has in many other ways assisted me in the preparation of this Volume.

In Volume II the Systematic treatment of Plants will be concluded, and the last chapters will be devoted to such subjects as geological floras, plants as rock-builders, fossil plants and evolution, and other general questions connected with Palaeobotany.

A. C. SEWARD.

Botanical Laboratory, Cambridge.
March, 1898.

TABLE OF CONTENTS.

LIST OF ILLUSTRATIONS.

[Frontispiece.] Tree Stumps in a Carboniferous Forest. Drawn from a photograph. (M. Seward.) [Page 57].

Note. The references in the footnotes require a word of explanation. The titles of the works referred to will be found in the Bibliography at the end of the volume. In this list the authors’ names are arranged alphabetically and the papers of each author are in chronological order. The numbers in brackets after the author’s name in the footnotes, and before his name in the bibliographical list, refer to the year of publication. Except in cases where the works were published prior to 1800, the first two figures are omitted: thus Ward (84) refers to a paper published by L. F. Ward in 1884. This system was suggested by Dr H. H. Field in the Biologisches Centralblatt, vol. XIII. 1893, p. 753. (Ueber die Art der Abfassung naturwissenschaftlicher Litteraturverzeichnisse.)

PART I. GENERAL.

CHAPTER I.

HISTORICAL SKETCH.

“But particular care ought to be had not to consult or take relations from any but those who appear to have been both long conversant in these affairs, and likewise persons of Sobriety, Faithfulness and Discretion, to avoid the being misled and imposed upon either by falsehood, or the ignorance, credulity, and fancifulness, that some of these people are but too obnoxious unto.” John Woodward, 1728.

The scientific study of fossil plants dates from a comparatively recent period, and palaeobotany has only attained a real importance in the eyes of botanists and geologists during the last few decades of the present century. It would be out of place, in a short treatise like the present, to attempt a detailed historical sketch, or to give an adequate account of the gradual rise and development of this modern science. An excellent Sketch of Palaeobotany has recently been drawn up by Prof. Lester Ward[1], of the United States Geological Survey, and an earlier historical retrospect may be found in the introduction to an important work by an eminent German palaeobotanist, the late Prof. Göppert[2]. In the well-known work by Parkinson on The Organic Remains of a Former World[3] there is much interesting information as to the early history of our knowledge of fossil plants, as well as a good exposition of the views held at the beginning of this century.

FOSSIL PLANTS AND THE FLOOD.

As a means of bringing into relief the modern development of the science of fossil plants, we may briefly pass in review some of the earlier writers, who have concerned themselves in a greater or less degree with a descriptive or speculative treatment of the records of a past vegetation. In the early part of the present century, and still more in the eighteenth century, the occurrence of fossil plants and animals in the earth’s crust formed the subject of animated, not to say acrimonious, discussion. The result was that many striking and ingenious theories were formulated as to the exact manner of formation of fossil remains, and the part played by the waters of the deluge in depositing fossiliferous strata. The earlier views on fossil vegetables are naturally bound up with the gradual evolution of geological science. It is from Italy that we seem to have the first glimmering of scientific views; but we are led to forget this early development of more than three hundred years ago, when we turn to the writings of English and other authors of the eighteenth century. “Under these white banks by the roadside,” as a writer on Verona has expressed it, “was born, like a poor Italian gipsy, the modern science of geology.” Early in the sixteenth century the genius of Leonardo da Vinci[4] compelled him to adopt a reasonable explanation of the occurrence of fossil shells in rocks far above the present sea-level. Another Italian writer, Fracastaro, whose attention was directed to this matter by the discovery of numerous shells brought to light by excavations at Verona, expressed his belief in the organic nature of the remains, and went so far as to call in question the Mosaic deluge as a satisfactory explanation of the deposition of fossil-bearing strata.

The partial recognition by some observers of the true nature of fossils marks the starting point of more rational views. The admission that fossils were not mere sports of nature, or the result of some wonderful ‘vis lapidifica,’ was naturally followed by numerous speculations as to the manner in which the remains of animals and plants came to be embedded in rocks above the sea-level. For a long time, the ‘universal flood’ was held responsible by nearly all writers for the existence of fossils in ancient sediments. Dr John Woodward, in his Essay toward a Natural History of the Earth, propounded the somewhat revolutionary theory, that “the whole terrestrial globe was taken all to pieces and dissolved at the Deluge, the particles of stone, marble, and all solid fossils dissevered, taken up into the water, and there sustained together with sea-shells and other animal and vegetable bodies: and that the present earth consists, and was formed out of that promiscuous mass of sand, earth, shells, and the rest falling down again, and subsiding from the water[5].” In common with other writers, he endeavoured to fix the exact date of the flood by means of fossil plants. Speaking of some hazel-nuts, which were found in a Cheshire moss pit, he draws attention to their unripened condition, and adds: “The deluge came forth at the end of May, when nuts are not ripe.” As additional evidence, he cites the occurrence of “Pine cones in their vernal state,” and of some Coal-Measure fossils which he compares with Virginian Maize, “tender, young, vernal, and not ripened[6].” Woodward (1665–1728) was Professor of Physic in Gresham College; he bequeathed his geological collections to the University of Cambridge, and founded the Chair which bears his name.

Another writer, Mendes da Costa, in a paper in the Philosophical Transactions for 1758, speaks of the impressions of “ferns and reed-like plants” in the coal-beds, and describes some fossils (Sigillaria and Stigmaria) as probably unknown forms of plant life[7].

Here we have the suggestion that in former ages there were plants which differed from those of the present age. Discussing the nature of some cones (Lepidostrobi) from the ironstone of Coalbrookdale in Shropshire, he concludes: “I firmly believe these bodies to be of vegetable origin, buried in the strata of the Earth at the time of the universal deluge recorded by Moses.” Scheuchzer of Zurich, the author of one of the earliest works on fossil plants and a “great apostle of the Flood Theory,” figures and describes a specimen as an ear of corn, and refers to its size and general appearance as pointing to the month of May as the time of the deluge[8]. Another English writer, Dr Parsons, in giving an account of the well-known ‘fossil fruits and other bodies found in the island of Sheppey,’ is disposed to dissent from Woodward’s views as to the time of the flood. He suggests that the fact of the Sheppey fruits being found in a perfectly ripe condition, points to the autumn as the more probable time for the occurrence of the deluge[9].

In looking through the works of the older writers, and occasionally in the pages of latter-day contributors, we frequently find curiously shaped stones, mineral markings on rock surfaces, or certain fossil animals, described as fossil plants. In Plot’s Natural History of Oxfordshire, published in 1705, a peculiarly shaped stone, probably a flint, is spoken of as one of the ‘Fungi lethales non esculenti[10]’; and again a piece of coral[11] is compared with a ‘Bryony root broken off transversely.’ On the other hand, that we may not undervalue the painstaking and laborious efforts of those who helped to lay the foundations of modern science, we may note that such authors as Scheuchzer and Woodward were not misled by the moss-like or dendritic markings of oxide of manganese on the surface of rocks, which are not infrequently seen to-day in the cabinets of amateurs as specimens of fossil plants.

The oldest figures of fossil plants from English rocks which are drawn with any degree of accuracy are those of Coal-Measure ferns and other plants in an important work by Edward Lhwyd published at Oxford in 1760[12].

Passing beyond these prescientific speculations, brief reference may be made to some of the more eminent pioneers of palaeobotany. The Englishman Artis[13] deserves mention for the quality rather than the quantity of his contributions to Palaeozoic botany; and among American authors Steinhauer’s[14] name must hold a prominent place in the list of those who helped to found this branch of palaeontology. Among German writers, Schlotheim stands out prominently as one who first published a work on fossil plants which still remains an important book of reference. Writing in 1804, he draws attention to the neglect of fossils from a scientific standpoint; they are simply looked upon, he says, as “unimpeachable documents of the flood[15].” His book contains excellent figures of many Coal-Measure plants, and we find in its pages occasional comparisons of fossil species with recent plants of tropical latitudes. Among the earlier authors whose writings soon become familiar to the student of fossil plants, reference must be made to Graf Sternberg, who was born three years before Schlotheim, but whose work came out some years later than that of the latter. His great contribution to Fossil Botany entitled Versuch einer geognostisch-botanischen Darstellung der Flora der Vorwelt, was published in several parts between the years 1820 and 1838; it was drawn up with the help of the botanist Presl, and included a valuable contribution by Corda[16]. In addition to descriptions and numerous figures of plants from several geological horizons, this important work includes discussions on the formation of coal, with observations on the climates of past ages.

STERNBERG AND BRONGNIART.

Sternberg endeavoured to apply to fossil plants the same methods of treatment as those made use of in the case of recent species. About the same time as Sternberg’s earlier parts were published, Adolphe Brongniart[17] of Paris began to enrich palaeobotanical science by those splendid researches which have won for him the title of the “Father of palaeobotany.” In Brongniart’s Prodrome, and Histoire des végétaux fossiles, and later in his Tableau des genres de végétaux fossiles, we have not merely careful descriptions and a systematic arrangement of the known species of fossil plants, but a masterly scientific treatise on palaeobotany in its various aspects, which has to a large extent formed the model for the best subsequent works on similar lines. From the same author, at a later date, there is at least one contribution to fossil plant literature which must receive a passing notice even in this short sketch. In 1839 he published an exhaustive account of the minute structure of one of the well-known Palaeozoic genera, Sigillaria; this is not only one of the best of the earliest monographs on the histology of fossil species, but it is one of the few existing accounts of the internal structure of this common type[18]. The fragment of a Sigillarian stem which formed the subject of Brongniart’s memoir is in the Natural History Museum in the Jardin des Plantes, Paris. It affords a striking example of the perfection of preservation as well as of the great beauty of the silicified specimens from Autun, in Central France. Brongniart was not only a remarkably gifted investigator, whose labours extend over a period connecting the older and more crude methods of descriptive treatment with the modern development of microscopic analysis, but he possessed the power of inspiring a younger generation with a determination to keep up the high standard of the palaeobotanical achievements of the French School. In some cases, indeed, his disciples have allowed a natural reverence for the Master to warp their scientific judgement, where our more complete knowledge has naturally led to the correction of some of Brongniart’s conclusions. Without attempting to follow the history of the science to more recent times, the names of Heer, Lesquereux, Zigno, Massalongo, Saporta and Ettingshausen should be included among those who rendered signal service to the science of fossil plants. The two Swiss writers, Heer[19] and Lesquereux[20], contributed numerous books and papers on palaeobotanical subjects, the former being especially well known in connection with the fossil floras of Switzerland and of Arctic lands, and the latter for his valuable writings on the fossil plants of his adopted country, North America. Zigno[21] and Massalongo[22] performed like services for Italy, and the Marquis of Saporta’s name will always hold an honourable and prominent position in the list of the pioneers of scientific palaeobotany; his work on the Tertiary and Mesozoic floras of France being specially noteworthy among the able investigations which we owe to his ability and enthusiasm[23]. In Baron Ettingshausen[24] we have another representative of those students of ancient vegetation who have done so much towards establishing the science of fossil plants on a philosophical basis.

As in other fields of Natural Science, so also in a marked degree in fossil botany, a new stimulus was given to scientific inquiry by the application of the microscope to palaeobotanical investigation. In 1828 Sprengel published a work entitled Commentatio de Psarolithis, ligni fossilis genere[25]; in which he dealt in some detail with the well-known silicified fern-stems of Palaeozoic age, from Saxony, basing his descriptions on the characteristics of anatomical structure revealed by microscopic examination.

THE INTERNAL STRUCTURE OF FOSSIL PLANTS.

In 1833 Henry Witham of Lartington brought out a work on The Internal Structure of Fossil Vegetables[26]; this book, following the much smaller and less important work by Sprengel, at once established palaeobotany on a firmer scientific basis, and formed the starting point for those more accurate methods of research, which have yielded such astonishing results in the hands of modern workers. In the introduction Witham writes, “My principal object in presenting this work to the public, is to impress upon geologists the advantage of attending more particularly to the intimate organization of fossil plants; and should I succeed in directing their efforts towards the elucidation of this obscure subject, I shall feel a degree of satisfaction which will amply repay my labour[27].”

On another page he writes as follows,—“From investigations made by the most active and experienced botanical geologists, we find reason to conclude that the first appearance of an extensive vegetation occurred in the Carboniferous series; and from a recent examination of the mountain-limestone groups and coal-fields of Scotland, and the north of England, we learn that these early vegetable productions, so far from being simple in their structure, as had been supposed, are as complicated as the phanerogamic plants of the present day. This discovery necessarily tends to destroy the once favourite idea, that, from the oldest to the most recent strata, there has been a progressive development of vegetable and animal forms, from the simplest to the most complex[28].” Since Witham’s day we have learnt much as to the morphology of Palaeozoic plants, and can well understand the opinions to which he thus gives expression.

It would be difficult to overrate the immense importance of this publication from the point of view of modern palaeobotany.

The art of making transparent sections of the tissues of fossil plants seems to have been first employed by Sanderson, a lapidary, and it was afterwards considerably improved by Nicol[29]. This most important advance in methods of examination gave a new impetus to the subject, but it is somewhat remarkable that the possibilities of the microscopical investigation of fossil plants have been but very imperfectly realised by botanical workers until quite recent years. As regards such a flora as that of the Coal-Measures, we can endorse the opinion expressed at the beginning of the century in reference to the study of recent mosses—“Ohne das Göttergeschenk des zusammengesetzten Mikroskops ist auf diesem Felde durchaus keine Ernte[30].” A useful summary of the history of the study of internal structure is given by Knowlton in a memoir published in 1889[31]. Not long after Witham’s book was issued there appeared a work of exceptional merit by Corda[32], in which numerous Palaeozoic plants are figured and fully described, mainly from the standpoint of internal structure. This author lays special stress on the importance of studying the microscopical structure of fossil plants.

ENGLISH PALAEOBOTANISTS.

Without pausing to enumerate the contributions of such well-known continental authors as Göppert, Cotta, Schimper, Stenzel, Schenk and a host of others, we may glance for a moment at the services rendered by English investigators to the study of palaeobotanical histology. Unfortunately we cannot always extend our examination of fossil plants beyond the characters of external form and surface markings; but in a few districts there are preserved remnants of ancient floras in which fragments of stems, roots, leaves and other structures have been petrified in such a manner as to retain with wonderful completeness the minute structure of their internal tissues. During the deposition of the coal seams in parts of Yorkshire and Lancashire the conditions of fossilisation were exceptionally favourable, and thus English investigators have been fortunately placed for conducting researches on the minute anatomy of the Coal-Measure plants. The late Mr Binney of Manchester did excellent service by his work on the internal structure of some of the trees of the Coal Period forests. In his introductory remarks to a monograph on the genus Calamites, after speaking of the desirability of describing our English specimens, he goes on to say, “When this is done, we are likely to possess a literature on our Carboniferous fossils worthy of the first coal-producing country[33].” The continuation and extension of Binney’s work in the hands of Carruthers, Williamson, and others, whose botanical qualifications enabled them to produce work of greater scientific value, has gone far towards the fulfilment of Binney’s prophecy.

DIFFICULTIES OF IDENTIFICATION.

In dealing with the structure of Palaeozoic plants, we shall be under constant obligation to the splendid series of memoirs from the pen of Prof. Williamson[34]. As the writer of a sympathetic obituary notice has well said: “In his fifty-fifth year he began the great series of memoirs which mark the culminating point of his scientific activity, and which will assure to him, for all time, in conjunction with Brongniart, the honourable title of a founder of modern Palaeobotany[35].” If we look back through a few decades, and peruse the pages of Lindley and Hutton’s classic work[36] on the Fossil flora of Great Britain, a book which is indispensable to fossil botanists, and read the description of such a genus as Sigillaria or Stigmaria; or if we extend our retrospect to an earlier period and read Woodward’s description of an unusually good specimen of a Lepidodendron, and finally take stock of our present knowledge of such plants, we realise what enormous progress has been made in palaeobotanical studies. Lindley and Hutton, in the preface to the first volume of the Flora, claim to have demonstrated that both Sigillaria and Stigmaria were plants with “the highest degree of organization, such as Cactaeae, or Euphorbiaceae, or even Asclepiadeae”; Woodward describes his Lepidodendron (Fig. 1) as “an ironstone, black and flat, and wrought over one surface very finely, with a strange cancellated work[37].” Thanks largely to the work of Binney, Carruthers, Hooker, Williamson, and to the labours of continental botanists, we are at present almost as familiar with Lepidodendron and several other Coal-Measure genera as with the structure of a recent forest tree. While emphasizing the value of the microscopic methods of investigation, we are not disposed to take such a hopeless view of the possibilities of the determination of fossil forms, in which no internal structure is preserved, as some writers have expressed. The preservation of minute structure is to be greatly desired from the point of view of the modern palaeobotanist, but he must recognise the necessity of making such use as he can of the numberless examples of plants of all ages, which occur only in the form of structureless casts or impressions.

Fig. 1. Four leaf-cushions of a Lepidodendron. Drawn from a specimen in the Woodward Collection, Cambridge. (Nat. size.)

In looking through the writings of the earlier authors we cannot help noticing their anxiety to match all fossil plants with living species; but by degrees it was discovered that fossils are frequently the fragmentary samples of extinct types, which can be studied only under very unfavourable conditions. In the absence of those characters on which the student of living plants relies as guides to classification, it is usually impossible to arrive at any trustworthy conclusions as to precise botanical affinity. Brongniart and other authors recognised this fact, and instituted several convenient generic terms of a purely artificial and provisional nature, which are still in general use. The dangers and risks of error which necessarily attend our attempts to determine small and imperfect fragments of extinct species of plants, will be briefly touched on in another place.

CHAPTER II.

RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY.

“La recherche du plan de la création, voilà le but vers lequel nos efforts peuvent tendre aujourd’hui.” Gaudry, 1883.

Since the greater refinements and thoroughness of scientific methods and the enormous and ever-increasing mass of literature have inevitably led to extreme specialisation, it is more than ever important to look beyond the immediate limits of one’s own subject, and to note its points of contact with other lines of research. A palaeobotanist is primarily concerned with the determination and description of fossil plants, but he must at the same time constantly keep in view the bearing of his work on wider questions of botanical or geological importance. From the nature of the case, we have in due measure to adapt the methods of work to the particular conditions before us. It is impossible to follow in the case of all fossil species precisely the same treatment as with the more complete and perfect recent plants; but it is of the utmost importance for a student of palaeobotany, by adhering to the methods of recent botany, to preserve as far as he is able the continuity of the past and present floras. Palaeontological work has often been undertaken by men who are pure geologists, and whose knowledge of zoology or botany is of the most superficial character, with the result that biologists have not been able to avail themselves, to any considerable extent, of the records of extinct forms of life. They find the literature is often characterised by a special palaeontological phraseology, and by particular methods of treatment, which are unknown to the student of living plants and animals. From this and other causes a purely artificial division has been made between the science of the organic world of to-day and that of the past.

NEGLECT OF FOSSILS BY BOTANISTS.

Fossils are naturally regarded by a stratigraphical geologist as records which enable him to determine the relative age of fossil-bearing rocks. For such a purpose it is superfluous to inquire into the questions of biological interest which centre round the relics of ancient floras. Primarily concerned, therefore, with fixing the age of strata, it is easy to understand how geologists have been content with a special kind of palaeontology which is out of touch with the methods of systematic zoology or botany. On the other hand, the botanist whose observations and researches have not extended beyond the limits of existing plants, sees in the vast majority of fossil forms merely imperfect specimens, which it is impossible to determine with any degree of scientific accuracy. He prefers to wait for perfect material; or in other words, he decides that fossils must be regarded as outside the range of taxonomic botany. It would seem, then, that the unsatisfactory treatment or comparative neglect of fossil plants, has been in a large measure due to the narrowness of view which too often characterises palaeobotanical literature. This has at once repelled those who have made a slight effort to recognise the subject, and has resulted in a one-sided and, from a biological standpoint, unscientific treatment of this branch of science. It must be admitted that palaeobotanists have frequently brought the subject into disrepute by their over-anxiety to institute specific names for fragments which it is quite impossible to identify. This over-eagerness to determine imperfect specimens, and the practice of drawing conclusions as to botanical affinity without any trustworthy evidence, have naturally given rise to considerable scepticism as to the value of palaeobotanical records. Another point, which will be dealt with at greater length in a later chapter, is that geologists have usually shown a distinct prejudice against fossil plants as indices of geological age; this again, is no doubt to a large extent the result of imperfect and inaccurate methods of description, and of the neglect of and consequent imperfect acquaintance with fossil plants as compared with fossil animals.

The student of fossil plants should endeavour to keep before him the fact that the chief object of his work is to deal with the available material in the most natural and scientific manner; and by adopting the methods of modern botany, he should always aim to follow such lines as may best preserve the continuity of past and present types of plants. Descriptions of floras of past ages and lists of fossil species, should be so compiled that they may serve the same purpose to a stratigraphical geologist, who is practically a geographer of former periods of the Earth’s history, as the accounts of existing floras to students of present day physiography. The effect of carrying out researches on some such lines as these, should be to render available to both botanists and geologists the results of the specialist’s work.

In some cases, palaeobotanical investigations may be of the utmost service to botanical science, and of little or no value to geology. The discovery of a completely preserved gametophyte of Lepidodendron or Calamites, or of a petrified Moss plant in Palaeozoic rocks would appeal to most botanists as a matter of primary importance, but for the stratigraphical geologist such discoveries would possess but little value. On the other hand the discovery of some characteristic species of Coal-Measure plants from a deep boring through Mesozoic or Tertiary strata might be a matter of special geological importance, but to the botanist it would be of no scientific value. In very many instances, however, if the palaeobotanist follows such lines as have been briefly suggested, the results of his labours should be at once useful and readily accessible to botanists and geologists. As Humboldt has said in speaking of Palaeontology, “the analytical study of primitive animal and vegetable life has taken a double direction; the one is purely morphological, and embraces especially the natural history and physiology of organisms, filling up the chasms in the series of still living species by the fossil structures of the primitive world. The second is more specially geognostic, considering fossil remains in their relations to the superposition and relative age of the sedimentary formations[38].”

FOSSIL PLANTS AND DISTRIBUTION.

To turn for a moment to some of the most obvious connections between palaeobotany and the wider sciences of botany and geology. The records of fossil species must occupy a prominent position in the data by which we may hope to solve some at least of the problems of plant evolution. From the point of view of distribution, palaeobotany is of considerable value, not only to the student of geographical botany, but to the geologist, who endeavours to map out the positions of ancient continents with the help of palaeontological evidence. The present distribution of plants and animals represents but one chapter in the history of life on the Earth; and to understand or appreciate the facts which it records, we have to look back through such pages as have been deciphered in the earlier chapters of the volume. The distribution of fossil plants lies at the foundation of the principles of the present grouping of floras on the Earth’s surface. Those who have confined their study of distribution to the plant geography of the present age, must supplement their investigations by reference to the work of palaeobotanical writers. If the lists of plant species drawn up by specialists in fossil botany, have been prepared with a due sense of the important conclusions which botanists may draw from them from the standpoint of distribution, they will be readily accepted as sound links in the chain of evidence. Unfortunately, however, if many of the lists of ancient floras were made use of in such investigations, the conclusions arrived at would frequently be of little value on account of the untrustworthy determinations of many of the species. In the case of particular genera the study of the distribution of the former species both in time and space, that is geologically and geographically, points to rational explanations of, or gives added significance to, the facts of present day distribution. That isolated conifer, Ginkgo biloba L. now restricted to Japan and China, was in former times abundant in Europe and in other parts of the world. It is clearly an exceedingly ancient type, isolated not only in geographical distribution but in botanical affinities, which has reached the last stage in its natural life. The Mammoth trees of California (Sequoia sempervirens Endl., and S. gigantea Lindl. and Gord.) afford other examples of a parallel case. The Tulip tree of North America and China and other allied forms are fairly common in the Tertiary plant beds of Europe, but the living representatives are now exclusively North American. Such differences in distribution as are illustrated by these dicotyledonous forest trees in Tertiary times and at the present day, have been clearly explained with the help of the geological record. Forbes, Darwin, Asa Gray[39] and others have been able to explain many apparent anomalies in the distribution of existing plants, and to reconcile the differences between the past and present distribution of many genera by taking account of the effect on plant life of the glacial period. As the ice gradually crept down from the polar regions and spread over the northern parts of Europe, many plants were driven further south in search of the necessary warmth. In the American continent such migration was rendered possible by the southern land extension; in Europe on the other hand the southerly retreat was cut off by impassable barriers, and the extinction of several genera was the natural result.

The comparatively abundant information which we possess as to the past vegetation of polar regions and the value of such knowledge to geologists and botanists alike is in striking contrast to the absence of similar data as regards Antarctic fossils. Darwin in an exceedingly interesting letter to Hooker à propos of a forthcoming British Association address, referring to this subject writes as follows:—

“The extreme importance of the Arctic fossil plants is self-evident. Take the opportunity of groaning over our ignorance of the Lignite plants of Kerguelen Land, or any Antarctic land. It might do good[40].”

In working out any collection of fossil plants, it would be well, therefore, to bear in mind that our aim should be rather to reproduce an accurate fragment of botanical history, than to perform feats of determination with hopelessly inadequate specimens. Had this principle been generally followed, the number of fossil plant species would be enormously reduced, but the value of the records would be considerably raised.

FOSSIL PLANTS AND CLIMATE.

Our knowledge of plant anatomy, and of those laws of growth which govern certain classes of plants to-day and in past time, has been very materially widened and extended by the facts revealed to us by the detailed study of Coal-Measure species. The modern science of Plant Biology, refounded by Charles Darwin, has thrown considerable light on the laws of plant life, and it enables us to correlate structural characteristics with physiological conditions of growth. Applying the knowledge gained from living plants to the study of such extinct types as permit of close microscopic examination, we may obtain a glimpse into the secrets of the botanical binomics of Palaeozoic times. The wider questions of climatic conditions depend very largely upon the evidence of fossil botany for a rational solution. As an instance of the best authenticated and most striking alternation in climatic conditions in comparatively recent times, we may cite the glacial period or Ice-Age. The existence of Arctic conditions has been proved by purely geological evidence, but it receives additional confirmation, and derives a wider importance from the testimony of fossil plants. In rocks deposited before the spread of ice from high northern latitudes, we find indubitable proofs of a widely distributed subtropical flora in Central and Northern Europe. Passing from these rocks to more recent beds there are found indications of a fall in temperature, and such northern plants as the dwarf Birch, the Arctic Willow and others reveal the southern extension of Arctic cold to our own latitudes.

The distribution of plants in time, that is the range of classes, families, genera and species of plants through the series of strata which make up the crust of the earth, is a matter of primary importance from a botanical as well as from a geological point of view.

Among the earlier writers, Brongniart recognised the marked differences between the earlier and later floras, and attempted to correlate the periods of maximum development of certain classes of plants with definite epochs of geological history. He gives the following classification in which are represented the general outlines of plant development from Palaeozoic to Tertiary times[41].

Since Brongniart’s time this method of classification has been extended to many of the smaller subdivisions of the geological epochs, and species of fossil plants are often of the greatest value in questions of correlation. In recent years the systematic treatment of Coal-Measure and other plants in the hands of various Continental and English writers has clearly demonstrated their capabilities for the purpose of subdividing a series of strata into stages and zones[42]. The more complete becomes our knowledge of any flora, the greater possibility there is of making use of the plants as indices of geological age[43].

FOSSIL PLANTS AND PHYLOGENY.

Not only is it possible to derive valuable aid in the correlation of strata from the facts of plant distribution, but we may often follow the various stages in the history of a particular genus as we trace the records of its occurrence through the geologic series. In studying the march of plant life through past ages, the botanist may sometimes follow the progress of a genus from its first appearance, through the time of maximum development, to its decline or extinction. In the Palaeozoic forests there was perhaps no more conspicuous or common tree than the genus long known under the name of Calamites. This plant attained a height of fifty or a hundred feet, with a proportionate girth, and increased in thickness in a manner precisely similar to that in which our forest trees grow in diameter. The exceptionally favourable conditions under which specimens of calamitean plants have been preserved, have enabled us to become almost as familiar with the minute structure of their stems and roots, as well as with their spore-producing organs, as with those of a living species. In short, it is thoroughly established that Calamites agrees in most essential respects with our well known Equisetum, and must be included in the same order, or at least sub-class, as the recent genus of Equisetaceae. As we ascend the geologic series from the Coal-Measures, a marked numerical decline of Calamites is obvious in the Permian period, and in the red sandstones of the Vosges, which belong to the same series of rocks as the Triassic strata of the Cheshire plain, the true Calamites is replaced by a large Equisetum apparently identical in external appearance and habit of growth with the species living to-day. In the more recent strata the Horse-tails are still represented, but the size of the Tertiary species agrees more closely with the comparatively small forms which have such a wide geographical distribution at the present time. Thus we are able to trace out the history of a recent genus of Vascular Cryptogams, and to follow a particular type of organisation from the time of its maximum development, through its gradual transition to those structural characters which are represented in the living descendants of the arborescent Calamites of the coal-period forests. The pages of such a history are frequently imperfect and occasionally missing, but others, again, are written in characters as clear as those which we decipher by a microscopical examination of the tissues of a recent plant.

As one of the most striking instances in which the microscopic study of fossil plants has shown the way to a satisfactory solution of the problems of development, we may mention such extinct genera as Lyginodendron, Myeloxylon and others. Each of these genera will be dealt with at some length in the systematic part of the book, and we shall afterwards discuss the importance of such types, from the point of view of plant evolution.

The botanist who would trace out the phylogeny of any existing class or family, makes it his chief aim to discover points of contact between the particular type of structure which he is investigating, and that of other more or less closely related classes or families.

Confining himself to recent forms, he may discover, here and there, certain anatomical or embryological facts, which suggest promising lines of inquiry in the quest after such affinities as point to a common descent. Without recourse to the evidence afforded by the plants of past ages, we must always admit that our existing classification of the vegetable kingdom is an expression of real gaps which separate the several classes of plants from one another. On the other hand our recently acquired and more accurate knowledge of such genera as have been alluded to, has made us acquainted with types of plant structure which enable us to fill in some of the lacunae in our existing classification. In certain instances we find merged in a single species morphological characteristics which, in the case of recent plants, are regarded as distinctive features of different subdivisions. It has been clearly demonstrated that in Lyginodendron, we have anatomical peculiarities typical of recent cycads, combined with structural characteristics always associated with existing ferns. In rare cases, it happens that the remarkably perfect fossilisation of the tissues of fossil plants, enables us not only to give a complete description of the histology of extinct forms, but also to speak with confidence as to some of those physiological processes which governed their life.

GEOLOGICAL HISTORY.

So far, palaeobotany has been considered in its bearings on the study of recent plants. From a geological point of view the records of ancient floras have scarcely less importance. In recent years, facts have been brought to light, which show that plants have played a more conspicuous part than has usually been supposed as agents of rock-building. As tests of geologic age, there are good grounds for believing that the inferiority of plants to animals is more apparent than real. This question, however, must be discussed at greater length in a later chapter.

Enough has been said to show the many-sided nature of the science of Fossil Plants, and the wide range of the problems which the geologist or botanist may reasonably expect to solve, by means of trustworthy data afforded by scientific palaeobotanical methods.

CHAPTER III.

GEOLOGICAL HISTORY.

“But how can we question dumb rocks whose speech is not clear[44]?”

In attempting to sketch in briefest outline the geological history of the Earth, the most important object to keep in view is that of reproducing as far as possible the broad features of the successive stages in the building of the Earth’s crust. It is obviously impossible to go into any details of description, or to closely follow the evolution of the present continents; at most, we can only refer to such facts as may serve as an introduction of the elements of stratigraphical geology to non-geological readers. For a fuller treatment of the subject reference must be made to special treatises on geology.

For the sake of convenience, it is customary in stratigraphical geology as also in biology, to make use of our imperfect knowledge as an aid to classification. If we possessed complete records of the Earth’s history, we should have an unbroken sequence, not merely of the various forms of life that ever existed, but of the different kinds of rocks formed in the successive ages of past time. As gaps exist in the chain of life, so also do we find considerable breaks in the sequence of strata which have been formed since the beginning of geologic time. The danger as well as the convenience of artificial classification must be kept in view. This has been well expressed by Freeman, in speaking of architectural styles,—“Our minds,” he says, “are more used to definite periods; they neglect or forget transitions which do indeed exist[45].” The idea of definite classification is liable to narrow our view of uniformity and the natural sequence of events.

ROCK-BUILDING.

Composing that part of the earth which is accessible to us,—or as it is generally called the earth’s crust,—there are rocks of various kinds, of which some have been formed by igneous agency, either as lavas or beds of ashes, or in the form of molten magmas which gradually cooled and became crystalline below a mass of superincumbent strata. With these rocks we need not concern ourselves.

A large portion of the earth’s crust consists of such materials as sandstones, limestones, shales, and similar strata which have been formed in precisely the same manner as deposits are being accumulated at the present day. The whole surface of the earth is continually exposed to the action of destructive agencies, and suffers perpetual decay; it is the products of this ceaseless wear and tear that form the building materials of new deposits.

The operation of water in its various forms, of wind, changes of temperature, and other agents of destruction cannot be fully dealt with in this short summary.

A river flowing to the sea or emptying itself into an inland lake, carries its burden of gravel, sand, and mud, and sooner or later, as the rate of flow slackens, it deposits the materials in the river-bed or on the floor of the sea or lake.

Fragments of rock, chipped off by wedges of ice, or detached in other ways from the parent mass, find their way to the mountain streams, and if not too heavy are conveyed to the main river, where the larger pieces come to rest as more or less rounded pebbles. Such water-worn rocks accumulate in the quieter reaches of a swiftly flowing river, or are thrown down at the head of the river’s delta. If such a deposit of loose water-worn material became cemented together either by the consolidating action of some solution percolating through the general mass, or by the pressure of overlying deposits, there would be formed a hard rock made up of rounded fragments of various kinds of strata derived from different sources. Such a rock is known as a Conglomerate. The same kind of rock may be formed equally well by the action of the sea; an old sea-beach with the pebbles embedded in a cementing matrix affords a typical example of a coarse conglomerate. Plant remains are occasionally met with in conglomerates, but usually in a fragmentary condition.

From a conglomerate composed of large water-worn pebbles, to a fine homogeneous sandstone there are numerous intermediate stages. A body of water, with a velocity too small to carry along pebbles of rock in suspension or to roll them along the bed of the channel, is still able to transport the finer fragments or grains of sand, but as a further decrease in the velocity occurs, these are eventually deposited as beds of coarse or fine sand. The stretches of sand on a gradually shelving sea shore, or the deposits of the same material in a river’s delta, have been formed by the gradual wearing away and disintegration of various rocks, the detritus of which has been spread out in more or less regular beds on the floor of a lake or sea. Such accumulations of fine detrital material, if compacted or cemented together, become typical Sandstones.

In tracing beds of sandstone across a tract of country, it is frequently found that the character of the strata gradually alters; mud or clay becomes associated with the sandy deposit, until finally the sandstone is replaced by beds of dark coloured shale. Similarly the sandy detritus on the ocean floor, or in an inland lake, when followed further and further from the source from which the materials were derived, passes by degrees into argillaceous sand, and finally into sheets of dark clay or mud. The hardened beds of clay or fine grained mud become transformed into Shales. As a general rule, then, shales are rocks which have been laid down in places further from the land, or at a greater distance from the source of origin of the detrital material, than sandstones or conglomerates. The conglomerates, or old shingle beaches, usually occur in somewhat irregular patches, marking old shore-lines or the head of a river delta. Coarse sandstones, or grits, may occur in the form of regularly bedded strata stretching over a wide area; and shales or clays may be followed through a considerable extent of country. The finer material composing the clays and shales has been held longer in suspension and deposited in deeper water in widespread and fairly horizontal layers.

In some districts sandstones occur in which the individual grains show a well marked rounding of the angles, and in which fossils are extremely rare or entirely absent. The close resemblance of such deposits to modern desert sands suggests a similar method of formation; and there can be no doubt that in some instances there have been preserved the wind-worn desert sands of former ages. Aeolian or wind-formed accumulations, although by no means common, are of sufficient importance to be mentioned as illustrating a certain type of rock.

CALCAREOUS ROCKS.

The thick masses of limestone which form so prominent a feature in parts of England and Ireland, have been formed in a manner different from that to which sandstones and shales owe their origin. On the floor of a clear sea, too far from land to receive any water-borne sediment, there is usually in process of formation a mass of calcareous material, which in a later age may rise above the surface of the water as chalk or LIMESTONE. Those organisms living in the sea, which are enclosed either wholly or in part by calcareous shells, are agents of limestone-building; their shells constantly accumulating on the floor of the sea give rise in course of time to a thick mass of sediment, composed in great part of carbonate of lime. Some of the shells in such a deposit may retain their original form, the calcareous body may on the other hand be broken up into minute fragments which are still recognisable with the help of a microscope, or the shells and other hard parts may be dissolved or disintegrated beyond recognition, leaving nothing in the calcareous sediment to indicate its method of formation.

Not a few limestones consist in part of fossil corals, and owe their origin to colonies of coral polyps which built up reefs or banks of coral in the ancient seas.

In the white cliffs of Dover, Flamborough Head and other places, we have a somewhat different form of calcareous rock, which in part consists of millions of minute shells of Foraminifera, in part of broken fragments of larger shells of extinct molluscs, and to some extent of the remains of siliceous sponges. As a general rule, limestones and chalk rocks are ancient sediments, formed in clear and comparatively deep water, composed in the main of carbonate of lime, in some cases with a certain amount of carbonate of magnesium, and occasionally with a considerable admixture of silica.

In such rocks land-plants must necessarily be rare. There are, however, limestones which wholly or in part owe their formation to masses of calcareous algae, which grew in the form of submarine banks or on coral reefs. Occasionally the remains of these algae are clearly preserved, but frequently all signs of plant structure have been completely obliterated. Again, there occur limestone rocks formed by chemical means, and in a manner similar to that in which beds of travertine are now being accumulated.

Granites, basalts, volcanic lavas, tuffs, and other igneous rocks need not claim our attention, except in such cases as permit of plant remains being found in association with these materials. Showers of ashes blown from a volcano, may fall on the surface of a lake or sea and become mixed with sand and mud of subaerial origin. Streams of lava occasionally flow into water, or they may be poured from submarine vents, and so spread out on the ocean bed with strata of sand or clay.

Passing from the nature and mode of origin of the sedimentary strata to the manner of their arrangement in the Earth’s crust, we must endeavour to sketch in the merest outline the methods of stratigraphical geology. The surface of the Earth in some places stands out in the form of bare masses of rock, roughly hewn or finely carved by Nature’s tools of frost, rain or running water; in other places we have gently undulating ground with beds of rock exposed to view here and there, but for the most part covered with loose material such as gravel, sands, boulder clay and surface soil.

GEOLOGICAL SECTIONS.

In the flat lands of the fen districts, the peat beds and low-lying salt marshes form the surface features, and are the connecting links between the rock-building now in progress and the deposits of an earlier age. If we could remove all these surface accumulations of sand, gravel, peat and surface soil, and take a bird’s eye view of the bare surface of the rocky skeleton of the earth’s crust, we should have spread before us the outlines of a geological map. In some places fairly horizontal beds of rock stretching over a wide extent of country, in another the upturned edges of almost vertical strata form the surface features; or, again, irregular bosses of crystalline igneous rock occur here and there as patches in the midst of bedded sedimentary or volcanic strata. A map showing the boundaries and distribution of the rocks as seen at the surface, tells us comparatively little as to the relative positions of the different rocks below ground, or of the relative ages of the several strata. If we supplement this superficial view by an inspection of the position of the strata as shown on the walls of a deep trench cut across the country, we at once gain very important information as to the relative position of the beds below the earth’s surface. The face of a quarry, the side of a river bed or a railway cutting, afford HORIZONTAL SECTIONS or PROFILES which show whether certain strata lie above or below others, whether a series of rocks consists of parallel and regularly stratified beds, or whether the succession of the strata is interfered with by a greater or less divergence from a parallel arrangement. If, for example, a section shows comparatively horizontal strata lying across the worn down edges of a series of vertical sedimentary rocks, we may fairly assume that some such changes as the following have taken place in that particular area.

The underlying beds were originally laid down as more or less horizontal deposits; these were gradually hardened and compacted, then elevated above sea-level by a folding of the earth’s crust; the crests of the folds were afterwards worn down by denudation, and the eroded surface finally subsided below sea-level and formed the floor on which newer deposits were built up. Such breaks in the continuity of stratified deposits are known as UNCONFORMITIES; in the interval of time which they represent great changes took place of which the records are either entirely lost, or have to be sought elsewhere.

In certain more exceptional cases, it is possible to obtain what is technically known as a VERTICAL SECTION; for example if a deep boring is sunk through a series of rocks, and the core of the boring examined, we have as it were a sample of the earth’s crust which may often teach us valuable lessons which cannot be learnt from maps or horizontal sections.

INVERSION OF STRATA.

It is obvious, that in a given series of beds, which are either horizontal or more or less obliquely inclined, the underlying strata were the first formed, and the upper beds were laid down afterwards. If, however, we trusted solely to the order of superposition in estimating relative age, our conclusions would sometimes be very far from the truth. Recent geological investigations have brought to light facts well nigh incredible as to the magnitude and extent of rock-foldings. In regions of great earth-movements, the crust has been broken along certain lines, and great masses of strata have been thrust for miles along the tops of newer rocks. Thus it may be brought about that the natural sequence of a set of beds has been entirely altered, and older rocks have come to overlie sediments of a later geological age. Facts such as these clearly illustrate the difficulties of correct geological interpretation.

In the horizontal section (Fig. 2), from the summit of Büzistock on the left to Saasterg on the right, we have a striking case of intense rock-folding and dislocation[46]. Prof. Heim[47] of Geneva has given numerous illustrations of the almost incredible positions assumed in the Swiss Mountains by vast thicknesses of rocks, and in the accompanying section taken from a recent work by Rothpletz we have a compact example of the possibilities of earth-movements as an agent of rock-folding. The section illustrates very clearly an exception to the rule that the order of superposition of a set of beds indicates the relative age of the strata. The horizontal line at the base is drawn at a height of 1650 metres above sea-level, and the summit of Büzistock reaches a height of 2340 m. The youngest rocks seen in the diagram are the Eocene beds e, at the base and as small isolated patches on the right-hand end of the section; the main mass of material composing the higher ground has been bodily thrust over the Eocene rocks, and in this process some of the beds, b and c, have been folded repeatedly on themselves. Similar instances of the overthrusting of a considerable thickness of strata have been described in the North-west Highlands of Scotland[48] and elsewhere in the British Isles. It is important therefore to draw attention to cases of extreme folding, as such phenomena are by no means exceptional in many parts of the world.

Fig. 2. Section from Büzistock to Saasterg. [After Rothpletz, (94) Pl. II. fig. 2.]

1. Sernifit or Verrucano (Permian).
2. Röthidolomit etc. (Permian).
3. Dogger (Jurassic).
4. Malm (Jurassic).
5. Eocene.

The order of superposition of strata has afforded the key to our knowledge of the succession of life in geologic time, and the refinements of the stratigraphical correlation of sedimentary rocks are based on the comparison of their fossil contents. By a careful examination of the relics of fossil organisms obtained from rocks of all ages and countries, it has been found possible to restore in broken outline the past history of the Earth. By means, then, of stratigraphical and palaeontological evidence, a classification of the various rocks has been established, the lines of division being drawn in such places as represent gaps in the fossil records, or striking and widespread unconformities between different series of deposits.

It is only in a few regions that we find rocks which can reasonably be regarded as the foundation stones of the Earth. As the globe gradually cooled, and its molten mass became skinned over with a solid crust, crystalline rocks must have been produced before the dawn of life, and before water could remain in a liquid form on the rocky surface. As soon as the temperature became sufficiently low, running water and rain began the work of denudation and rock disintegration which has been ceaselessly carried on ever since. In this continual breaking down and building up of the Earth’s surface, it would be no wonder if but few remnants were left of the first formed sediments of the earliest age.

The action of heat, pressure and chemical change accompanying rock-foldings and crust-wrinklings, often so far alters sedimentary deposits, that their original form is entirely lost, and sandstone, shales and limestones become metamorphosed into crystalline quartzites, slates and marbles.

The operation of metamorphism is therefore another serious difficulty in the way of recognising the oldest rocks. The earliest animals and plants which have been discovered are not such as we should expect to find as examples of the first products of organic life. Below the oldest known fossiliferous rocks, there must have been thousands of feet of sedimentary material, which has either been altered beyond recognition, or from some cause or other does not form part of our present geological record.

As a general introduction to geological chronology, a short summary may be given of the different formations or groups of strata, to which certain names have been assigned to serve as convenient designations for succeeding epochs in the world’s evolution. The following table (Fig. 3, pp. 32, 33) represents the geological series in a convenient form; the most characteristic rocks of each period are indicated by the usual conventional shading, and the most important breaks or lacunae in the records are shown by gaps and uneven lines. The relative thickness of the rocks of each period is approximately shown; but the vertical extent of the oldest or Archaean rocks as shown in Fig. 3 represents what is without doubt but a fraction of their proportional thickness. This table is taken, with certain alterations, from a paper by Prof. T. McKenny Hughes in the Cambridge Philosophical Proceedings for 1879. Speaking of the graphic method of showing the geological series, the author of the paper says, “It is convenient to have a table of the known strata, and although we cannot arrange all the rocks of the world in parallel columns, and say that ABC of one area are exactly synchronous with A′B′C′ of another, still if we take any one country and establish a grouping for it, we find so many horizons at which equivalent formations can be identified in distant places, that we generally make an approximation to HOMOTAXIS as Huxley called it. The most convenient grouping is obviously to bracket together locally continuous deposits, i.e. all the sediment which was formed from the time when the land went down and accumulation began, to the time when the sea bottom was raised and the work of destruction began. In the accompanying table I have given the rocks of Great Britain classified on this system, and bearing in mind that waste in one place must be represented by deposit elsewhere, I have represented the periods of degradation by intervals estimated where possible by the amount of denudation known to have taken place between the periods of deposition in the same district[49].”

TABLE OF STRATA.

Fig. 3.

I. Archaean.

“Men can do nothing without the make-believe of a beginning.”
George Eliot.

There is perhaps no problem at once so difficult and so full of interest to the student of the Earth’s history, as the interpretation of the fragmentary records of the opening stages in geological and organic evolution. In tracing the growth and development of the human race, it becomes increasingly difficult to discover and decipher written documents as we penetrate farther back towards the beginning of the historical period; the records are usually incomplete and fragmentary, or rendered illegible by the superposed writings of a later date. So in the records of the rocks, as we pass beyond the oldest strata in which clearly preserved fossils are met with, we come to older rocks which afford either no data as to the period in which they were formed, or like the palimpsest, with its original characters almost obliterated by a late MS., the older portions of the Earth’s crust have been used and re-used in the rock-building of later ages. In the first place, it is exceedingly difficult to determine with any certainty what rocks may be regarded as trustworthy fragments of a primaeval land. Throughout the geological eras the Earth’s surface has been subjected to foldings and wrinklings, volcanic activity has been almost unceasing, and there is abundant evidence to show how the original characters of both igneous and sedimentary rocks may be entirely effaced by the operation of chemical and physical forces. It was formerly held that coarsely crystalline rocks such as granite are the oldest portions of the crust, but modern geology has conclusively proved that many of the so-called fundamental masses of rock are merely piles of ancient sediments which have been subjected to the repeated operation of powerful physical and chemical forces, and have undergone a complete rearrangement of their substance. As the result of more detailed investigations, many regions formerly supposed to consist of the foundation stones of the Earth’s crust, are now known to have been centres of volcanic disturbance and widespread metamorphism, and to be made up of post-archaean rocks.

THE OLDEST ROCKS.

The first formed rocks no doubt became at once the prey of denudation and disintegration, and on their surface would be accumulated the products of their own destruction: newer strata would entirely cover up portions of the original land, to be in their turn succeeded by still later deposits. There is reason to believe that in the remotest ages of the Earth’s history, the forces of denudation and igneous activity were more potent than in later times, and thus the oldest rocks could hardly retain their original structure through the long ages of geologic time. The earliest representatives of organic life were doubtless of such a perishable nature that their remains could not be preserved in a fossil state even under the most favourable conditions. Such organisms, whether plants or animals, as possessed any resistant tissues or hard skeletons might be preserved in the oldest rocks, but as these strata became involved in earth-foldings or were penetrated by injections of igneous eruptions, the relics of life would be entirely destroyed. It is, in short, practically hopeless to look for any fragments of the primitive crust except such as have undergone very considerable metamorphism, and equally futile to search for any recognisable remains of primitive life.

In many parts of the world vast thicknesses of rock occur below the oldest known fossiliferous strata; these consist largely of laminated crystalline masses composed of quartz, felspar, and other minerals, having in fact the same composition as granite, but differing in the regular arrangement of the constituent parts. To such rocks the terms gneiss and schist have been applied. Rocks of this kind are by no means always of Archaean age, but many of the earliest known rocks consist of gneisses of various kinds, associated with altered lavas, metamorphosed ashes, breccias and other products of volcanic activity; with these there may be limestones, shales, sandstones, and other strata more or less closely resembling sedimentary deposits. Such a succession of gneissic rocks has been described as occupying a wide area in the basin of the St Lawrence river, and to these enormously thick and widespread masses a late Director of the Canadian Geological Survey applied the term Laurentian. These Laurentian rocks, with similar strata in Scandinavia, the north-west Highlands of Scotland, in certain parts of such mountain ranges as the Alps, Pyrenees, Carpathians, Himalayas, Andes, Atlas, &c., have been classed together as members of the oldest geological period, and are usually referred to under the name of Archaean, or less frequently Azoic rocks. In some of the uppermost Archaean rocks there have been recently discovered a few undoubted traces of fossil animals, but with this exception no fossils are known throughout the great mass of Archaean strata. It is true that some authorities regard the beds of graphite and other rocks as a proof of the abundance of plant life, but this supposition is not supported by any convincing evidence.

The term Azoic[50] applied by some writers to these oldest rocks suggests the absence of life during the period in which they were formed. Life there must have been, though we are unable to discover its records. The period of time represented by the Archaean or Pre-Cambrian rocks must be enormous, and it was in that earliest era that the first links in the chain of life were forged.

II. Cambrian.

The term Cambrian was adopted by Sedgwick for a series of sedimentary rocks in North Wales (Cambria). In that district, in South Wales, the Longmynd Hills, the Malverns, in Scotland, and other regions there occur more or less highly folded and contorted beds of pebbly conglomerate, sandstones, shales and slates resting on the uneven surface of an Archaean foundation.

It is in these Cambrian rocks that trustworthy records of organic life are first met with. Among the most constant and characteristic fossils of this period are the extinct and aberrant members of the crustacea, the trilobites; these with some brachiopods, sponges, and other fossils comprise the oldest fauna, of which the ancestral types have yet to be discovered. During the last few decades the number of Cambrian fossils has been considerably increased, and in certain regions of North America and China there are found many thousand feet of strata above the typical Archaean rocks and below the newer fossiliferous beds of Cambrian age. It is reasonable to suppose that future research may extend the present limits of fossil-bearing rocks below the horizon, which is marked by the occurrence of the widely distributed and oldest known trilobite, the genus Olenellus.

The vast thickness of Cambrian strata was for the most part laid down on the floor of a comparatively deep sea; other members of the series represent the shingle beaches and coast deposits accumulated on the slopes of Archaean islands. There have been many conjectures as to the distribution of land and sea during the deposition of these rocks; but the data are too imperfect to enable us to restore with any degree of confidence the physical geography of this Palaeozoic epoch, of which the sediments stood out as islands of Cambrian land during many succeeding ages.

III. Ordovician.

Since the days when Sedgwick and Murchison first worked out the succession of Palaeozoic strata in North Wales, there has always existed a considerable difference of opinion as to the best method of subdividing the Cambrian-Silurian strata. Later research has shown that the rocks included by Sedgwick in his Cambrian system, fall naturally into two groups; for the upper of these Prof. Lapworth has suggested the term Ordovician, from the name of the Ordovices, who inhabited a part of northern Wales. At the base of the system we have a series of volcanic and sedimentary rocks to which Sedgwick gave the name Arenig; above these there occur the Llandeilo Flags, succeeded by a considerable thickness of rocks known as the Bala series. The rocks making up these Ordovician sediments consist for the most part of slates, sandstones and limestones with volcanic ashes and lavas. Much of the typical Welsh scenery owes its character to the folded and weathered rocks laid down on the floor of the Ordovician sea, on which from many centres of volcanic activity lava streams and showers of ash were spread out between sheets of marine sediment. The Arenig Hills, Snowdonia, and many other parts of North and South Wales, parts of Shropshire, Scotland, Sweden, Russia, Bohemia, North America and other regions consist of great thicknesses of Ordovician strata.

IV. Silurian.

Passing up a stage higher in the geologic series, we have a succession of conglomerates, sandstones, shales, and limestones; in other words, a series of beds which represent pebbly shore deposits, the sands and muds of deeper water, and the accumulated débris of calcareous skeletons of animals which lived in the clear water of the Silurian sea. The term Silurian (Siluria was the country of Caractacus and the old Britons known as Silures[51]) was first applied by Murchison in 1835 to a more comprehensive series of rocks than are now included in the Silurian system. The rocks of this period occur in Wales, Shropshire, parts of Scotland, Ireland, Scandinavia, Russia, the United States and other countries. After the accumulation of the thick Ordovician sediments, the sea-floor was upraised and in places converted into ridges or islands of land, of which the detritus formed part of the material of Silurian deposits. The limestones of the Wenlock ridge have yielded an abundant fauna, consisting of corals, crinoids, molluscs and other invertebrates. In this period we have the first representatives of the Vertebrata, discovered in the rocks of Ludlow. In fact, in the Silurian period, “all the great divisions of the Animal Kingdom were already represented[52].”

V. Devonian.

By the continued elevation of the Silurian sea-floor, large portions became dry land, and during the succeeding period most of the British area formed part of a continental mass. Over the southern part of England, there still lay an arm of the sea, and in this were laid down the marine sediments which now form part of Devon, and from which the name Devonian has been taken as a convenient designation for the strata of this period. In parts of the northern land, in the region now occupied by Scotland, there were large inland lakes, on the floor of which vast thicknesses of shingle beds and coarse sands (“Old Red Sandstone”) were slowly accumulated; and it has been shown by Sir Archibald Geikie and others that during this epoch there were considerable outpourings of volcanic material in the Scotch area.

Farther to the West and South-west there was another large lake in which the so-called Kiltorkan beds of Ireland were deposited. In these Irish sediments, and others of the same age in Belgium and elsewhere a few forms of land plants have been discovered; but it is from the Devonian rocks of North America that most of our knowledge of the flora of this period has been obtained.

VI. Carboniferous.

From the point of view of palaeobotany, the shales, sandstones, and seams of coal included in the Carboniferous system are of special interest. It is from the relics of this Palaeozoic vegetation that the most important botanical lessons have been learnt.

The following classification of Carboniferous rocks shows the order of succession of the various beds, and the nature of the rocks which were formed at this stage in the Earth’s history.

In the classification of Carboniferous rocks adopted in Geikie’s text-book of Geology the following arrangement is followed for the Carboniferous limestone series[54]:—

The thick beds of mountain limestone, with their characteristic marine fossil shells and corals play an important part in English scenery. In Derbyshire, West Yorkshire, and other places, the limestone crags and hills are made up of the raised floor of a comparatively deep Carboniferous sea, which covered a considerable portion of the British Isles at the beginning of this epoch.

CARBONIFEROUS ROCKS.

The accumulation of the calcareous skeletons of marine animals, with masses of coral, veritable shell-banks of extinct oyster-like lamellibranchs, built up during the lapse of a long period of time, formed widespread deposits of calcareous sediments. These were eventually succeeded by less pure calcareous deposits, the sea became shallower, and land detritus found its way over an area formerly occupied by the clear waters of an open sea. The shallowing process was gradually continued, and the sea was by some means converted into a more confined fresh-water or brackish area, in which were laid down many hundred feet of coarse sandy sediments derived from the waste of granitic highlands. Finally the conditions became less constant; the continuous deposition of sandy detritus being interrupted by the more or less complete filling up of the area of sedimentation, and the formation of a land surface which supported a luxuriant vegetation, of which the débris was subsequently converted into beds of coal. By further subsidence the land was again submerged, and the forest-covered area became overspread with sands and muds.

Such are the imperfect outlines of the general physical conditions which are represented by the series of sedimentary strata included in the Carboniferous system. At the close of this period, the Earth’s surface in Western Europe was subjected to crust-foldings on a large scale, along lines running approximately North and South and East and West, the two sets of movements resulting in the formation of ridges of Carboniferous rocks. The uppermost series of grits, sandstones and coal-seams were in great part removed by denudation from the crests of the elevated ridges, but remained in the intervening troughs or basins where they were less exposed to denudation. It is the direct consequence of this, that we have our Coal-Measures preserved in the form of detached basins of upper Carboniferous beds.

A closer examination of the comparative thickness and succession of Carboniferous rocks in different parts of Britain shows very clearly that in the northern area of Scotland and in the North of England the conditions were different from those which obtained further South. Seeing how much palaeobotanical interest attaches to these rocks, it is important to treat a little more fully of their geology.

In parts of Devon, Cornwall and West Somerset, the Devonian strata are succeeded by a series of folded and contorted rocks which have yielded a comparatively small number of Carboniferous fossils. To this succession of limestones, shales and grits the term Culm-Measures was applied by Sedgwick and Murchison in 1837. The rocks of this series occupy a trough between the Devonian rocks of North and South Devon. While some authorities have correlated the Culm-Measures with the Millstone Grit, others regard them as representing a portion of the true Coal-Measures, as well as the Carboniferous and Lower Limestone Shale[55]. It has recently been shown that among the lower Culm strata there occur bands of ancient deep-sea sediments, consisting of beds of chert containing siliceous casts of various species of Radiolaria. There can be no doubt that the discovery of deep-sea fossils in this particular development of the British Carboniferous system leads to the conclusion that “while the massive deposits of the Carboniferous limestone—formed of the skeletons of calcareous organisms—were in process of growth in the seas to the North, there existed to the South-west a deeper ocean in which siliceous organisms predominated and formed these siliceous radiolarian rocks[56].”

The Upper Culm-Measures consist of conglomerates, grits, sandstones and shales with some plant remains and other fossils, and constitute a typical set of shallow water sediments. In Westphalia, the Harz region, Thuringia, Silesia and Moravia there are rocks corresponding to the Culm-Measures of Devon, and some of these have also afforded evidence of deep water conditions.

COAL-MEASURES.

S. W. England, S. Wales, Derbyshire and Yorkshire. In these districts the Carboniferous limestone reaches a considerable thickness; in the Mendips it has a thickness of 3000 feet, and in the Pennine chain of 4000 feet. At the base of this limestone series there occurs in the southern districts the so-called lower limestone shale, consisting of clays, shales and sandy beds. Above the limestone we have the Millstone grit and Coal-Measures; but in the Pennine district there is a series of rocks consisting of impure limestones and shales, intercalated between the Millstone grit and Carboniferous limestone; for this group of rocks the term Yoredale series has been proposed. In the Isle of Man and Derbyshire sheets of lava are interbedded with the calcareous sediments, affording clear proof of submarine volcanic eruptions.

N. England and Scotland. In the Carboniferous rocks of Northumberland we have distinct indications of a shallower sea. The regular succession of limestone strata in West Yorkshire and other districts, gives place to a series of thinner beds of limestones, interstratified with shales and impure calcareous rocks. We have come within the range of land detritus which was spread out on the floor of a shallow sea. The lowest portion of the Mountain limestone is here represented by about 200 feet of shales and other rocks grouped together in the Tuedian series. The Upper Carboniferous limestone and Yoredale rocks of Yorkshire are represented by sandstones, carbonaceous limestones and some seams of coal, included in the Bernician series. Further north, again, another classification has been proposed for the still more aberrant succession of rocks; the lowest being spoken of as the Calciferous sandstone, and the upper as the Carboniferous limestone. The calciferous sandstone may be compared with the lower limestone shale and part of the Carboniferous limestone of England. The Carboniferous limestone of Scotland probably represents the upper part of the limestone of England and the Yoredale rocks of the Pennine and other areas.

Turning to the upper members of the Carboniferous system—in the Coal-Measures, as they were called in 1817 by William Smith,—we have a series of coal seams, sandstones, shales, and ironstones occurring for the most part in basin-shaped areas. As a general rule, each seam of coal, which varies in thickness from one inch to thirty feet, rests on a characteristic unstratified argillaceous rock known as Underclay.

The accompanying diagram (Fig. 4) illustrates the frequent intercalation of small bands of argillaceous and sandy rocks associated with the seams of coal.

The usual classification adopted for the British Coal-Measures is that of Upper, Middle, and Lower Coal-Measures; between the Upper and Middle divisions there occur certain transition or passage beds which are known as the Transition series. Continental writers, and more recently Mr Kidston of Stirling, have attempted with considerable success to correlate the Coal-producing strata by means of fossil plants[57].

Vertical section of the Bassey or Salts Coal seam, Rushton Colliery, Blackburn (Lower Coal-Measures). From a specimen 4 feet 4 inches in height, presented by Mr P. W. Pickup to the Manchester Museum, Owens College.

Finally, some reference must be made to the occurrence of Carboniferous rocks underneath more recent strata. In a geological map, or bird’s-eye view of a country, we see such rocks as appear at the surface; by means of deep borings, however, we are occasionally enabled to follow the course of older beds a considerable distance below the usually accessible part of the Earth’s crust. In the neighbourhood of London, Dover, and other places we have Tertiary and Mesozoic strata forming the surface of the country, but below these comparatively recent formations, the sinking of deep wells and other borings have proved the existence of a ridge of Palaeozoic rocks stretching from the South Wales Coal-field through the South-east of England to northern France, Belgium and Westphalia. It is from rocks forming part of this old ridge that characteristic Coal-Measure plants have been obtained from the Dover boring. In Fig. 5 is shown an almost complete pinnule of Neuropteris Scheuchzeri Hoffm., a well-known fern, marking a definite horizon of Upper Carboniferous rocks[58]. The small hairs on the pinnules, shown in the figure as fine lines lying more or less parallel to the midrib and across the lateral veins, are a characteristic feature of this species.

Fig. 5.

Imperfect pinnule of Neuropteris Scheuchzeri Hoffm., showing the characteristic hairs as fine lines traversing the lateral veins. From a specimen obtained from the Dover boring and now in the British Museum. Nat. size.

VII. Permian.

Reference has already been made to the earth-foldings which marked the close of Carboniferous times; “the open Mediterranean sea of the Carboniferous period in Europe was converted into a large inland sea, like the Caspian of the present day, surrounded by a rocky and hilly continent, on which grew trees and plants of various kinds[59].” In parts of

Lancashire, Westmoreland, the Eden Valley, and in the East of England from Sunderland to Nottingham, there occurs a succession of limestones, sandstones, clays and other rocks with occasional beds of rock-salt and gypsum, which represent the various forms of sediment and chemical precipitates formed on the floor of Permian lakes. The poverty of the fauna and flora of Permian strata points to conditions unfavourable to life; and there can be little doubt that the characteristic red rocks of St Bees Head, and the creamy limestones of the Durham coast are the upraised sediments of an inland salt-water lake. The term Dyas was proposed by Marcou for this series of strata as represented in Germany, where the rocks are conveniently grouped in two series, the Magnesian limestone or Zechstein and the red sandstones or Rothliegendes. The older and better known name of Permian was instituted by Murchison for the rocks of this age, from their extreme development in the old kingdom of Permia in Russia. Unfortunately considerable confusion has arisen from the employment of different names for rocks of the same geological period; and the grouping of the beds varies in different parts of the world. It is of interest to note, that in the Tyrol, Carinthia, and other places there are found patches of old marine beds which were originally laid down in an open sea, which extended over the site of the Mediterranean, into Russia and Asia. In Bohemia, the Harz district, Autun in Burgundy, and other regions, there are seams of Permian coal interstratified with the marls and sands. From these last named beds many fossil plants have been obtained, and important palaeobotanical facts brought to light by the investigations of continental workers. Volcanic eruptions, accompanied by lava streams and showers of ash, have been recognised in the Permian rocks of Scotland, and elsewhere.

In North America, Australia, and India the term Permo-Carboniferous is often made use of in reference to the continuous and regular sequence of beds which were formed towards the close of the Carboniferous and into the succeeding Permian epoch. The enormous series of freshwater Indian rocks, to which geologists have given the name of the Gondwana system, includes the sediments of more than one geological period, some of the older members being regarded as Permo-Carboniferous in age. These Indian beds, with others in Australia, South Africa, and South America, are of special interest on account of the characteristic southern hemisphere plants which they have afforded, and from the association with the fossiliferous strata of extensive boulder beds pointing to widespread glacial conditions.

VIII. Trias.

As we ascend the geologic series, and pass up to the rocks overlying the Permian deposits, there are found many indications of a marked change in the records of animal and plant life. Many of the characteristic Palaeozoic fossils are no longer represented, and in their place we meet with fresh and in many cases more highly differentiated organisms. The threefold division of the rocks of this period which suggested the term Trias to those who first worked out the succession of the strata, is typically illustrated over a wide area in Germany, in which the lowest or Bunter series is followed by the calcareous Muschelkalk, and this again by the clays, rock-salt, and sandstones of the Keuper series. In the Cheshire plain and in the low ground of the Midlands, we have a succession of red sandstones, conglomerates, and layers of rock-salt which correspond to the Bunter and Keuper beds of German geologists. These Triassic rocks were obviously formed in salt-water lakes, in which from time to time long continued evaporation gave rise to extensive deposit of rock-salt and other minerals. From the fact that it is this type of Triassic sediments which was first made known, it is often forgotten that the British and German rocks are not the typical representatives of this geological period. The ‘Alpine’ Trias of the Mediterranean region, in Asia, North America, and other countries, has a totally different facies, and includes limestones and dolomites of deep-sea origin. “The widespread Alpine Trias is the pelagic facies of the formation; the more restricted German Trias, on the other hand, is a shallow shore, bay or inland sea formation[60].”

In the Keuper beds of southern Sweden there are found workable seams of coal, and the beds of this district have yielded numerous well-preserved examples of the Triassic flora. A more impure coal occurs in the lower Keuper of Thuringia and S.-W. Germany, and to this group of rocks the term Lettenkohle is occasionally applied.

In the Rhaetic Alps of Lombardy, in the Tyrol, and in England, from Yorkshire to Lyme Regis, Devonshire, Somersetshire, and other districts there are certain strata at the top of the Triassic system known as the Rhaetic or Penarth beds. The uppermost Rhaetic beds, often described as the White Lias, afford evidence of a change from the salt lakes of the Trias to the open sea of the succeeding Jurassic period. Passing beyond this period of salt lakes and wind-swept barren tracts of land, we enter on another phase of the earth’s history.

IX. Jurassic.

The Jura mountains of western Switzerland consist in great part of folded and contorted rocks which were originally deposited on the floor of a Jurassic sea. In England the Jurassic rocks are of special interest, both for geological and historical reasons, as it is in them that we find a rich fauna and flora of Mesozoic age, and it was the classification of these beds by means of their fossil contents that gained for William Smith the title of the Father of English Geology. A glance at a geological map of England shows a band of Jurassic rocks stretching across from the Yorkshire coast to Dorset. These are in a large measure calcareous, argillaceous, and arenaceous sediments of an open sea; but towards the upper limit of the series, both freshwater and terrestrial beds are met with. Numerous fragments of old coral reefs, sea-urchins, crinoids, and other marine fossils are especially abundant; in the freshwater beds and old surface-soils, as well as in the marine sandstones and shales, we have remnants of an exceedingly rich and apparently tropical vegetation. This was an age of Reptiles as well as an age of Cycads. An interesting feature of these widely distributed Jurassic strata is the evidence they afford of distinct climatal zones; there are clear indications, according to the late Dr Neumayr, of a Mediterranean, a middle European, and a Boreal or Russian province[61]. The subdivisions of the English Jurassic rocks are as follows[62]:—

In tracing the several groups across England, and into other parts of Europe, their characters are naturally found to vary considerably; in one area a series is made up of typical clear water or comparatively deep sea sediments, and in another we have shallow water and shore deposits of the same age. The Lias rocks have been further subdivided into zones by means of the species of Ammonites which form so characteristic a feature of the Jurassic fauna. In the lower Oolite strata there are shelly limestones, clays, sandstones, and beds of lignite and ironstone. Without discussing the other subdivisions of the Jurassic period, we may note that in the uppermost members there are preserved patches of old surface-soils exposed in the face of the cliffs of the Dorset coast and of the Isle of Portland.

X. Cretaceous.

In the south of England, and in some other districts, it is difficult to draw any definite line between the uppermost strata of the Jurassic and the lowest of the Cretaceous period. The rocks of the so-called Wealden series of Kent, Surrey, Sussex, and the Isle of Wight, are usually classed as Lower Cretaceous, but there is strong evidence in favour of regarding them as sediments of the Jurassic period. The Cretaceous rocks of England are generally speaking parallel to the Jurassic strata, and occupy a stretch of country from the east of Yorkshire and the Norfolk coast to Dorset in the south-west. The Chalk downs and cliffs represent the most familiar type of Cretaceous strata. In the white chalk with its numerous flints, we have part of the elevated floor of a comparatively deep sea, which extended in Cretaceous times over a large portion of the east and south-east of England and other portions of the European continent. On the bed of this sea, beyond the reach of any river-borne detritus, there accumulated through long ages the calcareous and siliceous remains of marine animals, to be afterwards converted into chalk and flints. At the beginning of the period, however, other conditions obtained, and there extended over the south-east of England, and parts of north and north-west Germany and Belgium, a lake or estuary in which were built up deposits of clay, sand and other material, forming the delta of one or more large rivers. For these sediments the name Wealden was suggested in 1828. Eventually the gradual subsidence of this area led to an incursion of the sea, and the delta became overflowed by the waters of a large Cretaceous sea. At first the sea was shallow, and in it were laid down coarse sands and other sediments known as the Lower Greensand rocks. By degrees, as the subsidence continued, the shallows became deep water, and calcareous material slowly accumulated, to be at last upraised as beds of white chalk. The distribution of fossils in the Cretaceous rocks of north and south Europe distinctly points to the existence of two fairly well-marked sets of organisms in the two regions; no doubt the expression of climatal zones similar to those recognised in Jurassic times. In North America, Cretaceous rocks are spread over a wide area, also in North Africa, India, South Africa, and other parts of the world. Within the Arctic Circle strata of this age have become famous, chiefly on account of the rich flora described from them by the Swiss palaeobotanist Heer. The fauna and flora of this epoch are alike in their advanced state of development and in the great variety of specific types; the highest class of plants is first met with at the base of the Cretaceous system.

XI. Tertiary.

“At the close of the Chalk age a change took place both in the distribution of land and water, and also in the development of organic life, so great and universal, that it has scarcely been equalled at any other period of the earth’s geological history[63].” The Tertiary period seems to bring us suddenly to the threshold of our own times. In England at least, the deposits of this age are of the nature of loose sands, clays and other materials containing shells, bones, and fossil plants bearing a close resemblance to organisms of the present era. The chalk rocks, upheaved from the Cretaceous sea, stood out as dry land over a large part of Britain; much of their material was in time removed by the action of denuding agents, and the rest gradually sank again beneath the waters of Tertiary lakes and estuaries. In the south of England, and in north Europe generally, the Tertiary rocks have suffered but little disturbance or folding, but in southern Europe and other parts of the world, the Tertiary sands have been compacted and hardened into sandstones, and involved in the gigantic crust-movements which gave birth to many of our highest mountain chains. The Alps, Carpathians, Apennines, Himalayas, and other ranges consist to a large extent of piled up and strangely folded layers of old Tertiary sediments. The volcanic activity of this age was responsible for the basaltic lavas of the Giants’ Causeway, the Isle of Staffa, and other parts of western Scotland.

During the succeeding phases of this period, the distribution of land and sea was continually changing, climatic conditions varied within wide limits; and in short wherever Tertiary fossiliferous beds occur, we find distinct evidence of an age characterised by striking activity both as regards the action of dynamical as well as of organic forces. Sir Charles Lyell proposed a subdivision of the strata of this period into Eocene, Miocene, and Pliocene, founding his classification on the percentage of recent species of molluscs contained in the various sets of rocks. His divisions have been generally adopted. In 1854 Prof. Beyrich proposed to include another subdivision in the Tertiary system, and to this he gave the name Oligocene.

Occupying a basin-shaped area around London and Paris there are beds of Eocene sands and clays which were originally deposited as continuous sheets of sediment in water at first salt, afterwards brackish and to a certain extent fresh. In the Hampshire cliffs and in some parts of the Isle of Wight, we have other patches of these oldest Tertiary sediments. Across the south of Europe, North Africa, Arabia, Persia, the Himalayas, to Java and the Philippine islands, there existed in early Tertiary times a wide sea connecting the Atlantic and Pacific oceans; and it may be that in the Mediterranean of to-day we have a remnant of this large Eocene ocean. Later in the Tertiary period a similar series of beds was deposited which we now refer to as the Oligocene strata; such occurs in the cliffs of Headon hill in the Isle of Wight, containing bones of crocodiles, and turtles, with the relics of a rich flora preserved in the delta deposits of an Oligocene river. At a still later stage the British area was probably dry land, and an open sea existed over the Mediterranean region. In the neighbourhood of Vienna we have beds of this age represented by a succession of sediments, at first marine and afterwards freshwater. Miocene beds occur over a considerable area in Switzerland and the Arctic regions, and they have yielded a rich harvest to palaeobotanical investigators.

On the coast of Essex, Suffolk, Norfolk, the south of Cornwall, and other districts there occur beds of shelly sand and gravel long known under the name of ‘Crag.’ The beds have a very modern aspect; the sands have not been converted into sandstones, and the shells have undergone but little change. These materials were for the most part accumulated on the bed of a shallow sea which swept over a portion of East Anglia in Pliocene times. In the sediments of this age northern forms of shells and other organisms make their appearance, and in the Cromer forest-bed there occur portions of drifted trees with sands, clays and gravels, representing in all probability the débris thrown down on the banks of an ancient river. At this time the greater part of the North Sea was probably a low-lying forest-covered region, through which flowed the waters of a large river, of which part still exists in the modern Rhine. The lowering of temperature which became distinctly pronounced in the Pliocene age, continued until the greater part of Britain and north Europe experienced a glacial period, and such conditions obtained as we find to-day in ice-covered Greenland. Finally the ice-sheet melted, the local glaciers of North Wales, the English Lake district and other hilly regions, retreated, and after repeated alterations in level, the land of Great Britain assumed its modern form. The submerged forests and peat beds familiar in many parts of the coast, the diatomaceous deposits of dried up lakes, “remain as the very finger touches of the last geological change.”

GEOLOGICAL EVOLUTION.

The agents of change and geological evolution, which we have passed in brief review, are still constantly at work carrying one step further the history of the earth. A superficial review of geological history gives us an impression of recurring and widespread convulsions, and rapidly effected revolutions in organic life and geographical conditions; on the other hand a closer comparison of the past and present, with due allowance for the enormous period of time represented by the records of the rocks, helps us to realise the continuity of geological evolution. “So that within the whole of the immense period indicated by the fossiliferous stratified rocks, there is assuredly not the slightest proof of any break in the uniformity of Nature’s operations, no indication that events have followed other than a clear and orderly sequence[64].”

CHAPTER IV.

THE PRESERVATION OF PLANTS AS FOSSILS.

“The things, we know, are neither rich nor rare,

But wonder how the devil they got there.”

Pope, Prologue to the Satires.

The discovery of a fossil, whether as an impression on the surface of a slab of rock or as a piece of petrified wood, naturally leads us back to the living plant, and invites speculation as to the circumstances which led to the preservation of the plant fragment. There is a certain fascination in endeavouring, with more or less success, to picture the exact conditions which obtained when the leaf or stem was carried along by running water and finally sealed up in a sedimentary matrix. Attempts to answer the question—How came the plant remains to be preserved as fossils?—are not merely of abstract interest appealing to the imagination, but are of considerable importance in the correct interpretation of the facts which are to be gleaned from the records of plant-bearing strata.

Before describing any specific examples of the commoner methods of fossilisation; we shall do well to briefly consider how plants are now supplying material for the fossils of a future age. In the great majority of cases, an appreciation of the conditions of sedimentation, and of the varied circumstances attending the transport and accumulation of vegetable débris, supplies the solution of a problem akin to that of the fly in amber and the manner in which it came there.

OLD SURFACE-SOILS.

Seeing that the greater part of the sedimentary strata have been formed in the sea, and as the sea rather than the land has been for the most part the scene of rock-building in the past, it is not surprising that fossil plants are far less numerous than fossil animals. With the exception of the algae and a few representatives of other classes of plants, which live in the shallow-water belt round the coast, or in inland lakes and seas, plants are confined to land-surfaces; and unless their remains are swept along by streams and embedded in sediments which are accumulating on the sea floor, the chance of their preservation is but small. The strata richest in fossil plants are often those which have been laid down on the floor of an inland lake or spread out as river-borne sediment under the waters of an estuary. Unlike the hard endo- and exo-skeletons of animals, the majority of plants are composed of comparatively soft material, and are less likely to be preserved or to retain their original form when exposed to the wear and tear which must often accompany the process of fossilisation.

The Coal-Measure rocks have furnished numberless relics of a Palaeozoic vegetation, and these occur in various forms of preservation in rocks laid down in shallow water on the edge of a forest-covered land. The underclays or unstratified argillaceous beds which nearly always underlie each seam of coal have often been described as old surface-soils, containing numerous remains of roots and creeping underground stems of forest trees. The overlying coal has been regarded as a mass of the carbonised and compressed débris of luxuriant forests which grew on the actual spot now occupied by the beds of coal. There are, however, many arguments in favour of regarding the coal seams as beds of altered vegetable material which was spread out on the floor of a lagoon or lake, while the underclay was an old soil covered by shallow water or possibly a swampy surface tenanted by marsh-loving plants[65].

The Jurassic beds of the Yorkshire Coast, long famous as some of the richest plant-bearing strata in Britain, and the Wealden rocks of the south coast afford examples of Mesozoic sediments which were laid down on the floor of an estuary or large lake. Circumstances have occasionally rendered possible the preservation of old land-surfaces with the stumps of trees still in their position of growth. One of the best examples of this in Britain are the so-called dirt-beds or black bands of Portland and the Dorset Coast. On the cliffs immediately east of Lulworth Cove, the surface of a ledge of Purbeck limestone which juts out near the top of the cliffs, is seen to have the form here and there of rounded projecting bosses or ‘Burrs’ several feet in diameter. In the centre of each boss there is either an empty depression, or the remnants of a silicified stem of a coniferous tree. Blocks of limestone 3 to 5 feet long and of about equal thickness may be found lying on the rocky ledge presenting the appearance of massive sarcophagi in which the central trough still contains the silicified remains of an entombed tree. The calcareous sediment no doubt oozed up to envelope the thick stem as it sank into the soft mud. An examination of the rock just below the bed bearing these curious circular elevations reveals the existence of a comparatively narrow band of softer material, which has been worn away by denuding agents more rapidly than the overlying limestone. This band consists of partially rounded or subangular stones associated with carbonaceous material, and probably marks the site of an old surface-soil. This old soil is well shown in the cliffs and quarries of Portland, and similar dirt-beds occur at various horizons in the Lower and Middle Purbeck Series[66]. In this case, then, we have intercalated in a series of limestone beds containing marine and freshwater shells two or three plant beds containing numerous and frequently large specimens of cycadean and coniferous stems, lying horizontally or standing in their original position of growth. These are vestiges of an ancient forest which spread over a considerable extent of country towards the close of the Jurassic period. The trunks of cycads, long familiar in the Isle of Portland as fossil crows’ nests, have usually the form of round depressed stems with the central portion somewhat hollowed out. It was supposed by the quarrymen that they were petrified birds’ nests which had been built in the forks of the trees which grew in the Portland forest. The beds separating the surface-soils of the Purbeck Series, as seen in the sections exposed on the cliffs or quarries, point to the subsidence of a forest-covered area over which beds of water-borne sediment were gradually deposited, until in time the area became dry land and was again taken possession of by a subtropical vegetation, to be once more depressed and sealed up under layers of sediment[67].

A still more striking example of the preservation of forest trees rooted in an old surface-soil is afforded by the so-called fossil-grove in Victoria Park, Glasgow, ([Frontispiece]). The stumps of several trees, varying in diameter from about one to three feet, are fixed by long forking ‘roots’ in a bed of shale. In some cases the spreading ‘roots,’ which bear the surface features of Stigmaria, extend for a distance of more than ten feet from the base of the trunk. The stem surface is marked by irregular wrinklings which suggest a fissured bark; but the superficial characters are very imperfectly preserved. In one place a flattened Lepidodendron stem, about 30 feet long, lies prone on the shale. Each of the rooted stumps is oval or elliptical in section, and the long axes of the several stems are approximately parallel, pointing to some cause operating in a definite direction which gave to the stems their present form. Near one of the trees, and at a somewhat higher level than its base, the surface of the rock is clearly ripple-marked, and takes us back to the time when the sinking forest trees were washed by waves which left an impress in the soft mud laid down over the submerged area. The stumps appear to be those of Lepidodendron trees, rooted in Lower Carboniferous rocks. From their manner of occurrence it would seem that we have in them a corner of a Palaeozoic forest in which Lepidodendra played a conspicuous part. The shales and sandstones containing the fossil trees were originally overlain by a bed of igneous rock which had been forced up as a sheet of lava into the hardened sands and clays[68].

Other examples of old surface-soils occur in different parts of the world and in rocks of various ages. As an instance of a land surface preserved in a different manner, reference may be made to the thin bands of reddish or brown material as well as clays and shale which occasionally occur between the sheets of Tertiary lava in the Western Isles of Scotland and the north-east of Ireland. In the intervals between successive outpourings of basaltic lava in the north-west of Europe during the early part of the Tertiary period, the heated rocks became gradually cooler, and under the influence of weathering agents a surface-soil was produced fit for the growth of plants. In some places, too, shallow lakes were formed, and leaves, fruits and twigs became embedded in lacustrine sediments, to be afterwards sealed up by later streams of lava. In the face of the cliff at Ardtun Head on the coast of Mull a leaf-bed is exposed between two masses of gravel underlying a basaltic lava flow; the impressions of the leaves of Gingko and other plants from the Tertiary sediments of this district are exceptionally beautiful and well preserved[69]. A large collection obtained by Mr Starkie Gardner may be seen in the British Museum.

In 1883 the Malayan island of Krakatoa, 20 miles from Sumatra and Java, was the scene of an exceptionally violent volcanic explosion. Two-thirds of the island were blown away, and the remnant was left absolutely bare of organic life. In 1886 it was found that several plants had already established themselves on the hardened and weathered crust of the Krakatoan rocks, the surface of the lavas having been to a large extent prepared for the growth of the higher plants by the action of certain blue-green algae which represent some of the lowest types of plant life[70]. We may perhaps assume a somewhat similar state of things to have existed in the volcanic area in north-west Europe, where the intervals between successive outpourings of lava are represented by the thin bands of leaf-beds and old surface-soils.

On the Cheshire Coast at Leasowe[71] and other localities, there is exposed at low water a tract of black peaty ground studded with old rooted stumps of conifers and other trees (fig. 6). There is little reason to doubt that at all events the majority of the trees are in their natural place of growth. The peaty soil on which they rest contains numerous flattened stems of reeds and other plants, and is penetrated by roots, probably of some aquatic or marshy plants which spread over the site of the forest as it became gradually submerged. A lower forest-bed rests directly on a foundation of boulder clay. Such submerged forests are by no means uncommon around the British coast; many of them belong to a comparatively recent period, posterior to the glacial age. In many cases, however, the tree stumps have been drifted from the places where they grew and eventually deposited in their natural position, the roots of the trees, in some cases aided by stones entangled in their branches, being heavier than the stem portion. There is a promising field for botanical investigation in the careful analysis of the floras of submerged forests; the work of Clement Reid, Nathorst, Andersson and others, serves to illustrate the value of such research in the hands of competent students.

Fig. 6. Part of a submerged Forest seen at low water on the Cheshire Coast at Leasowe. Drawn from a photograph.

The following description by Lyell, taken from his American travels, is of interest as affording an example of the preservation of a surface-soil:

“On our way home from Charleston, by the railway from Orangeburg, I observed a thin black line of charred vegetable matter exposed in the perpendicular section of the bank. The sand cast out in digging the railway had been thrown up on the original soil, on which the pine forest grew; and farther excavations had laid open the junction of the rubbish and the soil. As geologists, we may learn from this fact how a thin seam of vegetable matter, an inch or two thick, is often the only monument to be looked for of an ancient surface of dry land, on which a luxuriant forest may have grown for thousands of years. Even this seam of friable matter may be washed away when the region is submerged, and, if not, rain water percolating freely through the sand may, in the course of ages, gradually carry away the carbon[72].”

FOSSIL WOOD.

In addition to the remnants of ancient soils, and the preservation of plant fragments in rocks which have been formed on the floor of an inland lake or an estuary, it is by no means rare to find fossil plants in obviously marine sediments. In fig. 7 we have a piece of coniferous wood with the shell of an Ammonite (Aegoceras planicosta Sow.) lying on it; the specimen was found in the Lower Lias clay at Lyme Regis, and illustrates the accidental association of a drifted piece of a forest tree with a shell which marks at once the age and the marine character of the beds. Again in fig. 8 we have a block of flint partially enclosing a piece of coniferous wood in which the internal structure has been clearly preserved in silica. This specimen was found in the chalk, a deposit laid down in the clear and deep water of the Cretaceous sea. The wood must have floated for some time before it became water-logged and sank to the sea-floor. In the light coloured wood there occur here and there dark spots which mark the position of siliceous plugs b, b filling up clean cut holes bored by Teredos in the woody tissue. The wood became at last enclosed by siliceous sediment and its tissues penetrated by silica in solution, which gradually replaced and preserved in wonderful perfection the form of the original tissue. A similar instance of wood enclosed in flint was figured by Mantell in 1844 in his Medals of Creation[73].

Fig. 7. Aegoceras planicosta Sow. on a piece of coniferous wood, Lower Lias, Lyme Regis. From a specimen in the British Museum. Slightly reduced.

Fig. 8. Piece of coniferous wood in flint, from the Chalk, Croydon. Drawn from a specimen presented to the British Museum by Mr Murton Holmes. In the side view, shown above in the figure, the position of the wood is shown by the lighter portion, with holes, b, b, bored by Teredos or some other wood-eating animal. In the end view, below, the wood is seen as an irregular cylinder w, w, embedded in a matrix of flint. ⅓ Nat. size.

The specimen represented in fig. 9 illustrates the almost complete destruction of a piece of wood by some boring animal. The circular and oval dotted patches represent the filled up cavities made by a Teredo or some similar wood-boring animal.

Fig. 9. Piece of wood from the Red Crag of Suffolk, riddled with holes filled in with mud. From a specimen in the York Museum. ⅓ Nat. size.

CONDITIONS OF FOSSILISATION.

Before discussing a few more examples of fossils illustrating different methods of fossilisation, it may not be out of place to quote a few extracts from travellers’ narratives which enable us to realise more readily the circumstances and conditions under which plant remains have been preserved in the Earth’s crust.

In an account of a journey down the Rawas river in Sumatra, Forbes thus describes the flooded country:—

“The whole surface of the water was covered, absolutely in a close sheet, with petals, fruits and leaves, of innumerable species. In placid corners sometimes I noted a collected mass nearly half a foot deep, among which, on examination, I could scarcely find a leaf that was perfect, or that remained attached to its rightful neighbour, so that were they to become imbedded in some soft muddy spot, and in after ages to reappear in a fossil form they would afford a few difficult puzzles to the palaeontologist, both to separate and to put together[74].”

An interesting example of the mixture of plants and animals in sedimentary deposits is described by Hooker in his Himalayan Journals:—

“To the geologist the Jheels and Sunderbunds are a most instructive region, as whatever may be the mean elevation of their waters, a permanent depression of ten to fifteen feet would submerge an immense tract, which the Ganges, Burrampooter, and Soormah would soon cover with beds of silt and sand.

“There would be extremely few shells in the beds thus formed, the southern and northern divisions of which would present two very different floras and faunas, and would in all probability be referred by future geologists to widely different epochs. To the north, beds of peat would be formed by grasses, and in other parts temperate and tropical forms of plants and animals would be preserved in such equally balanced proportions as to confound the palaeontologist; with the bones of the long-snouted alligator, Gangetic porpoise, Indian cow, buffalo, rhinoceros, elephant, tiger, deer, bear, and a host of other animals, he would meet with acorns of several species of oak, pine-cones and magnolia fruits, rose seeds, and Cycas nuts, with palm nuts, screw-pines, and other tropical productions[75].”

In another place the same author writes:

“On the 12th of January, 1848, the Moozuffer was steaming amongst the low, swampy islands of the Sunderbunds.... Every now and then the paddles of the steamer tossed up the large fruits of Nypa fruticans, Thunb., a low stemless palm that grows in the tidal waters of the Indian Ocean, and bears a large head of nuts. It is a plant of no interest to the common observer, but of much to the geologist, from the nuts of a similar plant abounding in the Tertiary formations at the mouth of the Thames, having floated about there in as great profusion as here, till buried deep in the silt and mud that now forms the island of Sheppey[76].”

DRIFTING OF TREES.

Of the drifting of timber, fruits, &c., we find numerous accounts in the writings of travellers. Rodway thus describes the formation of vegetable rafts in the rivers of Northern British Guiana:—

“Sometimes a great tree, whose timber is light enough to float, gets entangled in the grass, and becomes the nucleus of an immense raft, which is continually increasing in size as it gathers up everything that comes floating down the river[77].”

The undermining of river banks in times of flood, and the transport of the drifted trees to be eventually deposited in the delta is a familiar occurrence in many parts of the world. The more striking instances of such wholesale carrying along of trees are supplied by Bates, Lyell and other writers. In his description of the Amazon the former writes:

“The currents ran with great force close to the bank, especially when these receded to form long bays or enseadas, as they are called, and then we made very little headway. In such places the banks consist of loose earth, a rich crumbling vegetable mould, supporting a growth of most luxuriant forest, of which the currents almost daily carry away large portions, so that the stream for several yards out is encumbered with fallen trees, whose branches quiver in the current[78].”

In another place, Bates writes:

“The rainy season had now set in over the region through which the great river flows; the sand-banks and all the lower lands were already under water, and the tearing current, two or three miles in breadth, bore along a continuous line of uprooted trees and islets of floating plants[79].”

The rafts of the Mississippi and other rivers described by Lyell afford instructive examples of the distant transport of vegetable material. The following passage is taken from the Principles of Geology;

“Within the tropics there are no ice-floes; but, as if to compensate for that mode of transportation, there are floating islets of matted trees, which are often borne along through considerable spaces. These are sometimes seen sailing at the distance of fifty or one hundred miles from the mouth of the Ganges, with living trees standing erect upon them. The Amazons, the Orinoco, and the Congo also produce these verdant rafts[80].”

After describing the enormous natural rafts of the Atchafalaya, an arm of the Mississippi, and of the Red river, Lyell goes on to say:

“The prodigious quantity of wood annually drifted down by the Mississippi and its tributaries is a subject of geological interest, not merely as illustrating the manner in which abundance of vegetable matter becomes, in the ordinary course of nature, imbedded in submarine and estuary deposits, but as attesting the constant destruction of soil and transportation of matter to lower levels by the tendency of rivers to shift their courses.... It is also found in excavating at New Orleans, even at the depth of several yards below the level of the sea, that the soil of the delta contains innumerable trunks of trees, layer above layer, some prostrate as if drifted, others broken off near the bottom, but remaining still erect, and with their roots spreading on all sides, as if in their natural position[81].”

The drifting of trees in the ocean is recorded by Darwin in his description of Keeling Island, and their action as vehicles for the transport of boulders is illustrated by the same account.

“In the channels of Tierra del Fuego large quantities of drift timber are cast upon the beach, yet it is extremely rare to meet a tree swimming in the water. These facts may possibly throw light on single stones, whether angular or rounded, occasionally found embedded in fine sedimentary masses[82].”

Fruits may often be carried long distances from land, and preserved in beds far from their original source. Whilst cruising amongst the Solomon Islands, the Challenger met with fruits of Barringtonia speciosa &c., 130–150 miles from the coast. Off the coast of New Guinea long lines of drift wood were seen at right angles to the direction of the river; uprooted trees, logs, branches, and bark, often floating separately.

“The midribs of the leaves of a pinnate-leaved palm were abundant, and also the stems of a large cane grass (Saccharum), like that so abundant on the shores of the great river in Fiji. Various fruits of trees and other fragments were abundant, usually floating confined in the midst of the small aggregations into which the floating timber was everywhere gathered.... Leaves were absent except those of the Palm, on the midrib of which some of the pinnæ were still present. The leaves evidently drop first to the bottom, whilst vegetable drift is floating from a shore; thus, as the débris sinks in the sea water, a deposit abounding in leaves, but with few fruits and little or no wood, will be formed near shore, whilst the wood and fruits will sink to the bottom farther off the land. Much of the wood was floating suspended vertically in the water, and most curiously, logs and short branch pieces thus floating often occurred in separate groups apart from the horizontally floating timber. The sunken ends of the wood were not weighted by any attached masses of soil or other load of any kind; possibly the water penetrates certain kinds of wood more easily in one direction with regard to its growth than the other, hence one end becomes water-logged before the other.... The wood which had been longest in the water was bored by a Pholas[83].”

The bearing of this account on the manner of preservation of fossils, and the differential sorting so frequently seen in plant beds, is sufficiently obvious.

As another instance of the great distance to which land plants may be carried out to sea and finally buried in marine strata, an observation by Bates may be cited. When 400 miles from the mouth of the main Amazons, he writes:

“We passed numerous patches of floating grass mingled with tree trunks and withered foliage. Amongst these masses I espied many fruits of that peculiar Amazonian tree the Ubussú Palm; this was the last I saw of the great river[84].”

The following additional extract from the narrative of the Cruise of H.M.S. Challenger illustrates in a striking degree the conflicting evidence which the contents of fossiliferous beds may occasionally afford; it describes what was observed in an excursion from Sydney to Berowra Creek, a branch of the main estuary or inlet into which flows the Hawkesbury river. It was impossible to say where the river came to an end and the sea began. The Creek is described as a long tortuous arm of the sea, 10 to 15 miles long, with the side walls covered with orchids and Platycerium. The ferns and palms were abundant in the lateral shady glens; marine and inland animals lived in close proximity.

“Here is a narrow strip of the sea water, twenty miles distant from the open sea; on a sandy shallow flat close to its head are to be seen basking in the sun numbers of sting-rays.... All over these flats, and throughout the whole stretch of the creek, shoals of Grey Mullet are to be met with; numerous other marine fish inhabit the creek. Porpoises chase the mullet right up to the commencement of the sand-flat. At the shores of the creek the rocks are covered with masses of excellent oysters and mussel, and other shell-bearing molluscs are abundant, whilst a small crab is to be found in numbers in every crevice. On the other hand the water is overhung by numerous species of forest trees, by orchids and ferns, and other vegetation of all kinds; mangroves grow only in the shallow bays. The gum trees lean over the water in which swim the Trygon and mullet, just as willows hang over a pool of carp. The sandy bottom is full of branches and stems of trees, and is covered in patches here and there by their leaves. Insects constantly fall in the water, and are devoured by the mullet. Land birds of all kinds fly to and fro across the creek, and when wounded may easily be drowned in it. Wallabies swim across occasionally, and may add their bones to the débris at the bottom. Hence here is being formed a sandy deposit, in which may be found cetacean, marsupial, bird, fish, and insect remains, together with land and sea shells, and fragments of a vast land flora; yet how restricted is the area occupied by this deposit, and how easily might surviving fragments of such a record be missed by future geological explorers![85]”

MEANING OF THE TERM ‘FOSSIL.’

The term ‘fossil’ suggests to the lay mind a petrifaction or a replacement by mineral matter of the plant tissues. In the scientific sense, a fossil plant, that is a plant or part of a plant whether in the form of a true petrifaction or a structureless mould or cast, which has been buried in the earth by natural causes, may be indistinguishable from a piece of recent wood lately fallen from the parent tree. In the geologically recent peat beds such little altered fossils (or sub-fossils) are common enough, and even in older rocks the more resistant parts of plant fragments are often found in a practically unaltered state. In the leaf impressions on an impervious clay, the brown-walled epidermis shows scarcely any indication of alteration since it was deposited in the soft mud of a river’s delta. Such fossil leaves are common in the English Tertiary beds, and even in Palaeozoic rocks it is not uncommon to find an impression of a plant on a bed of shale from which the thin brown epidermis may be peeled off the rock, and if microscopically examined it will be found to have retained intact the contours of the cuticularised epidermal cells. A striking example of a similar method of preservation is afforded by the so-called paper-coal of Culm age from the Province of Toula in Russia[86]. In the Russian area the Carboniferous or Permian rocks have been subjected to little lateral pressure, and unlike the beds of the same age in Western Europe, they have not been folded and compressed by widespread and extensive crust-foldings. Instead of the hard seams of coal there occur beds of a dark brown laminated material, made up very largely of the cuticles of Lepidodendroid plants.

From such examples we may naturally pass to fossils in which the plant structure has been converted into carbonaceous matter or even pure coal. This form of preservation is especially common in plant-bearing beds at various geological horizons. In other cases, again, some mineral solution, oxide of iron, talc, and other substances, has replaced the plant tissues. From the Coal-Measures of Switzerland Heer has figured numerous specimens of fern fronds and other plants in which the leaf form has been left on the dark coloured rock surface as a thin layer of white talcose material[87]. In the Buntersandstone of the Vosges and other districts the red imperfectly preserved impressions of plant stems and leaves are familiar fossils[88]; the carbonaceous substance of the tissues has been replaced by a brown or red oxide of iron.

INCRUSTATIONS.

Plants frequently occur in the form of incrustations; and in fact incrustations, which may assume a variety of forms, are the commonest kind of fossil. The action of incrusting springs, or as they are often termed petrifying springs, is illustrated at Knaresborough, in Yorkshire, and many other places where water highly charged with carbonate of lime readily deposits calcium carbonate on objects placed in the path of the stream.

The travertine deposited in this manner forms an incrustation on plant fragments, and if the vegetable substance is subsequently removed by the action of water or decay, a mould of the embedded fragment is left in the calcareous matrix. An instructive example of this form of preservation was described in 1868[89] by Sharpe from an old gravel pit near Northampton. He found in a section eight feet high (fig. 10), a mass of incrusted plants of Chara (a) resting on and overlain by a calcareous paste (c) and (d) made up of the decomposed material of the overlying rock, and this again resting on sand. The place where the section occurred was originally the site of a pool in which Stoneworts grew in abundance. Large blocks of these incrusted Charas may be seen in the fossil-plant gallery of the British Museum.

Fig. 10. Section of an old pool filled up with a mass of Chara. (From the Geol. Mag. vol. v. 1868, p. 563.)

In the Natural History Museum in the Jardin des Plantes, Paris, one of the table-cases contains what appear to be small models of flowers in green wax. These are in reality casts in wax of the moulds or cavities left in a mass of calcareous travertine, on the decay and disappearance of the encrusted flowers and other plant fragments[90]. This porous calcareous rock occurs near Sézanne in Southern France, and is of Eocene age[91]. The plants were probably blown on to the freshly deposited carbonate of lime, or they may have simply fallen from the tree on to the incrusting matrix; more material was afterwards deposited and the flowers were completely enclosed. Eventually the plant substance decayed, and as the matrix hardened moulds were left of the vegetable fragments. Wax was artificially forced into these cavities and the surrounding substance removed by the action of an acid, and thus perfect casts were obtained of Tertiary flowers.

Darwin has described the preservation of trees in Van Diemen’s land by means of calcareous substances. In speaking of beds of blown sand containing branches and roots of trees he says:

“The whole became consolidated by the percolation of calcareous matter; and the cylindrical cavities left by the decaying of the wood were thus also filled up with a hard pseudo-stalactitical stone. The weather is now wearing away the softer parts, and in consequence the hard casts of the roots and branches of the trees project above the surface, and, in a singularly deceptive manner, resemble the stumps of a dead thicket[92].”

As a somewhat analogous method of preservation to that in travertine, the occurrence of plants in amber should be mentioned. In Eocene times there existed over a region, part of which is now the North-east German coast, an extensive forest of conifers and other trees. Some of the conifers were rich in resinous secretions which were poured out from wounded surfaces or from scars left by falling branches. As these flowed as a sticky mass over the stem or collected on the ground, flowers, leaves, and twigs blown by the wind or falling from the trees, became embedded in the exuded resin. Evaporation gradually hardened the resinous substance until the plant fragments became sealed up in a mass of amber, in precisely the same manner in which objects are artificially preserved in Canada balsam. In many cases the amber acts as a petrifying agent, and by penetrating the tissues of a piece of wood it preserves the minute structural details in wonderful perfection[93]. Dr Thomas in an account of the amber beds of East Prussia in 1848, refers to the occurrence of large fossil trees; he writes:

“The continuous changes to which the coast is exposed, often bring to light enormous trunks of trees, which the common people had long regarded as the trunks of the amber tree, before the learned declared that they were the stems of palm trees, and in consequence determined the position of Paradise to be on the coast of East Prussia[94].”

CASTS OF TREES.

In 1887 an enormous fossil plant was discovered in a sandstone quarry at Clayton near Bradford[95]. The fossil was in the form of a sandstone cast of a large and repeatedly branched Stigmaria, and it is now in the Owens College Museum, where it was placed through the instrumentality of Prof. Williamson. The plant was found spread out in its natural position on the surface of an arenaceous shale, and overlain by a bed of hard sandstone identical with the material of which the cast is composed. Williamson has thus described the manner of formation of the fossil:

“It is obvious that the entire base of the tree became encased in a plastic material, which was firmly moulded upon these roots whilst the latter retained their organisation sufficiently unaltered to enable them to resist all superincumbent pressure. This external mould then hardened firmly, and as the organic materials decayed they were floated out by water which entered the branching cavity; at a still later period the same water was instrumental in replacing the carbonaceous elements by the sand of which the entire structure now consists[96].”

Although the branches have not been preserved for their whole length, they extend a distance of 29 feet 6 inches from right to left, and 28 feet in the opposite direction.

The fossil represented in fig. 1 (p. 10), from the collection of Dr John Woodward, affords a good example of a well-defined impression. The surface of the specimen, of which a cast is represented in fig. 1, shows very clearly the characteristic leaf-cushions and leaf-scars of a Lepidodendron. The stem was embedded in soft sand, and as the latter became hard and set, an impression was obtained of the external markings of the Lepidodendron. Decay subsequently removed the substance of the plant.

Fig. 11. Equisetites columnaris Brongn. From a specimen in the Woodwardian Museum, Cambridge. ⅓ nat. size.

In fig. 11 some upright stems of a fossil Horse-tail (Equisetites columnaris) from the Lower Oolite rocks near Scarborough, are seen in a vertical position in sandstone. On the surface of the fossils there is a thin film of carbonaceous matter, which is all that remains of the original plant substance; the stems were probably floated into their present position and embedded vertically in an arenaceous matrix. The hollow pith-cavity was filled with sand, and as the tissues decayed they became in part converted into a thin coaly layer. The vertical position of such stems as those in fig. 11 naturally suggests their preservation in situ, but in this as in many other cases the erect manner of occurrence is due to the settling down of the drifted plants in this particular position.

FOSSIL CASTS.

An example of Stigmaria drawn in fig. 12 further illustrates the formation of casts[97]. The outer surface with the characteristic spirally arranged circular depressions, represents the wrinkled bark of the dried plant; the smaller cylinder, on the left side of the upper end (fig. 12, 2, p) marks the position of the pith surrounded by the secondary wood, which has been displaced from its axial position. The pith decayed first, and the space was filled in with mud; somewhat later the wood and cortex were partially destroyed, and the rod of material which had been introduced into the pith-cavity dropped towards one side of the decaying shell of bark.

Fig. 12. Stigmaria ficoides Brongn. 1. Side view, showing wrinkled surface and the scars of appendages. 2. End view (upper) showing the displaced central cylinder; p, pith, x, xylem, r, medullary rays. 3. End view (lower). From a specimen in the Woodwardian Museum. ½ nat. size.

As the parenchymatous medullary rays readily decayed, the mud in the pith extended outwards between the segments of wood which still remained intact, and so spokes of argillaceous material were formed which filled the medullary ray cavities. The cortical tissues were decomposed, and their place taken by more argillaceous material. At one end of the specimen (fig. 12, 3) we find the wood has decayed without its place being afterwards filled up with foreign material. At the opposite end of the specimen, the woody tissue has been partially preserved by the infiltration of a solution containing carbonate of lime (fig. 12, 2).

Numerous instances have been recorded from rocks of various geological ages of casts of stems standing erect and at right angles to the bedding of the surrounding rock. These vertical trees occasionally attain a considerable length, and have been formed by the filling in by sand or mud of a pipe left by the decay of the stem. It is frequently a matter of some difficulty to decide how far such fossils are in the position of growth of the tree, or whether they are merely casts of drifted stems, which happen to have been deposited in an erect position. The weighting of floating trees by stones held in the roots, added to the greater density of the root wood, has no doubt often been the cause of this vertical position. In attempting to determine if an erect cast is in the original place of growth of the tree, it is important to bear in mind the great length of time that wood is able to resist decay, especially under water. The wonderful state of preservation of old piles found in the bed of a river, and the preservation of wooden portions of anchors of which the iron has been completely removed by disintegration, illustrate this power of resistance. In this connection, the following passage from Lyell’s travels in America is of interest. In describing the site of an old forest, he writes[98]:

“Some of the stumps, especially those of the fir tribe, take fifty years to rot away, though exposed in the air to alternations of rain and sunshine, a fact on which every geologist will do well to reflect, for it is clear that the trees of a forest submerged beneath the water, or still more, if entirely excluded from the air, by becoming imbedded in sediment, may endure for centuries without decay, so that there may have been ample time for the slow petrifaction of erect fossil trees in the Carboniferous and other formations, or for the slow accumulation around them of a great succession of strata.”

In another place, in speaking of the trees in the Great Dismal Swamp, Lyell writes:—“When thrown down, they are soon covered by water, and keeping wet they never decompose, except the sap wood, which is less than an inch thick[99].” We see, then, that trees may have resisted decay for a sufficiently long time to allow of a considerable deposition of sediment. It is very difficult to make any computation of the rate of deposition of a particular set of sedimentary strata, and, therefore, to estimate the length of time during which the fossil stems must have resisted decay.

PLANTS AND COAL.

The protective qualities of humus acids, apart from the almost complete absence of Bacteria[100] from the waters of Moor- or Peat-land, is a factor of great importance in the preservation of plants against decay for many thousands of years.

From examples of fossil stems or leaves in which the organic material has been either wholly or in part replaced by coal, we may pass by a gradual transition to a mass of opaque coal in which no plant structure can be detected. It is by no means uncommon to notice on the face of a piece of coal a distinct impression of a plant stem, and in some cases the coal is obviously made up of a number of flattened and compressed branches or leaves of which the original tissues have been thoroughly carbonised. A block of French coal, represented in fig. 13, consists very largely of laminated bands composed of the long parallel veined leaves of the genus Cordaites and of the bark of Lepidodendron, Sigillaria, and other Coal-Measure genera. The long rhizomes and roots below the coal are preserved as casts in the underclay.

In examining thin sections of coal, pieces of pitted tracheids or crushed spores are frequently met with as fragments of plant structures which have withstood decay more effectually than the bulk of the vegetable débris from which the coal was formed.

The coaly layer on a fossil leaf is often found to be without any trace of the plant tissues, but not infrequently such carbonised leaves, if treated with certain reagents and examined microscopically, are seen to retain the outlines of the epidermal cells of the leaf surface. If a piece of the Carbonaceous film detached from a fossil leaf is left for some days in a small quantity of nitric acid containing a crystal of chlorate of potash, and, after washing with water, is transferred to ammonia, transparent film often shows very clearly the outlines of the epidermal cell and the form of the stomata. Such treatment has been found useful in many cases as an aid to determination[101]. Prof. Zeiller informs me that he has found it particularly satisfactory in the case of cycadean leaves.