| Page | |
| On the Pre-cambrian Rocks of the British Isles. | [1] |
| Are There Traces of Glacial Man in the Trenton Gravels? | [15] |
| Geology As a Part of a College Curriculum. | [38] |
| The Nature of the Englacial Drift of the Mississippi Basin. | [47] |
| Studies for Students. | [61] |
| Editorials. | [85] |
| Reviews. | [91] |
| Analytical Abstracts of Current Literature. | [95] |
| Acknowledgments. | [101] |
THE
JOURNAL OF GEOLOGY
JANUARY-FEBRUARY, 1893.
[ON THE PRE-CAMBRIAN ROCKS OF THE BRITISH ISLES.]
During the last twenty years much has been written about the "pre-Cambrian" rocks of the British Isles. Unfortunately when attention began to be sedulously given to the study of these ancient formations, the problems of metamorphism were still a hundred fold more obscure than they have since become; the aid of the microscope had not been seriously and systematically adopted for the investigation of the crystalline schists, and geologists generally were still under the belief that the broad structure of these schists could be treated like those of the sedimentary rocks, and be determined by rapid traverses of the ground. We have now painfully discovered that these older methods of observation were extremely crude, and that the work performed in accordance with them is now of little interest or value save as a historical warning to future generations of geologists. Geological literature has meanwhile been burdened with numerous contributions which remain as a permanent incubus on our library shelves.
It may serve a useful purpose at the present time in possibly aiding those who are engaged in the study of the oldest rocks of North America, if I place before them, as briefly as possible, the main facts which in my opinion have now been satisfactorily proved regarding the corresponding rocks of Britain, and if I indicate at the same time some of the more probable inferences in those cases where the facts, at present known, do not warrant a definite conclusion.
It is obvious that in any effort to establish that a group of rocks is older than the very base of the sedimentary fossiliferous formations, we must somewhere find that group emerging from under the bottom of these formations. Until lithological characters are ascertained to be so distinctive and constant as to be comparable to fossil evidence for purposes of stratigraphical identification, we should not assume that detached areas of older rocks rising amid Palæozoic, Secondary or Tertiary formations are pre-Cambrian. We should, if possible, begin at the bottom of the Palæozoic systems and work backward, tracing each successive system or group as these rise from under each other, until we arrive at what appears to be the oldest traceable within the region of observation. It is clear that in the present state of knowledge we have no satisfactory means of identifying such successive systems in widely separated countries. All that can be attempted in the meantime is to ascertain the special types in each region, and to point out their general resemblances or contrasts to those of other regions. It is better to avoid confusion by refraining from applying the stratigraphical names adopted for the oldest rocks of one region to those of another geographically remote, though we may hope that eventually it may be possible to work out the equivalence of these local names.
In the British Isles, by much the most important region for the study of the oldest rocks is to be found in the north-west Highlands of Scotland. The very basement strata of the Cambrian system are there traceable for a distance of more than 100 miles, reposing with a strong unconformability upon all rocks of older date. They consist of dolomitic shales with Olenellus, resting upon a thick group of quartzites, full of annelid tubes. One of the most remarkable features of these ancient strata is the persistence of their component bands or zones which, though sometimes only a few feet thick, can be traced throughout the whole tract of country just referred to. For the study of the pre-Cambrian rocks this is an important point, for we can be quite certain that even where fossil evidence locally fails, the same basement members of the Cambrian system are persistent and lie directly upon the pre-Cambrian series.
Lewisian Gneiss. Ever since the researches of Murchison and Nicol in the north-west of Scotland, it has been known that two distinct systems of rock underlie the quartzites to which I have just alluded. Murchison regarded the upper of these as of Cambrian age, while he assigned the unconformable quartzites and limestones above it to the Lower Silurian period. But the recent discovery of the Olenellus zone intercalated conformably between the quartzites and the overlying limestones may be regarded as proving that all the rocks which underlie the quartzites and are separated from them by a strong unconformability must be pre-Cambrian. It is thus established beyond any reasonable doubt that two great pre-Cambrian systems of rock exist in the north-west of Scotland.
These two systems differ so entirely from each other that their respective areas can be defined with minute accuracy. The uppermost consists chiefly of dull reddish sandstones with conglomerates, and especially towards their base in Rosshire, some bands of dark grey shale, the whole having a thickness of at least 8,000 or 10,000 feet, though as both the base and the top of the series are marked by strong unconformabilities, the whole original thickness of deposits is nowhere seen. As these rocks are well developed around Loch Torridon, they were named by Nicol the Torridon Sandstone—a designation which has more recently been shortened into "Torridonian." The lower system is mainly composed of various foliated rocks which may be embraced under the general term "gneiss." These masses present the usual characters of the so-called "fundamental complex", "Urgebirge," or "Archæan Series" of other countries. The contrast between the thoroughly crystalline, gnarled, ancient-looking gneisses below, and the overlying, nearly horizontal Torridonian conglomerates, sandstones, and shales, which are largely made out of their debris, is so striking that every observer feels persuaded that in any logical system of classification they can not be both placed in the same division of the geological record. They are certainly both pre-Cambrian, but they must belong to widely separated eras, and must have been produced by entirely different processes. If it is proposed to regard the gneisses as "Archæan," we must refuse to include the Torridonian strata in the same section of pre-Cambrian time. But so much uncertainty exists as to the application of this term Archæan, examples are so multiplying wherein what was supposed to be the oldest and truly Archæan rock is found to be intrusive in rocks that were taken to be of much younger date, and there are such slender grounds for correlating the so-called Archæan rocks of one country with those of another, that I prefer for the present, at least, not to use the term at all. Let me very briefly state some of the main characteristics of the two sharply contrasted rock-systems of the north-west of Scotland.
The oldest gneiss of that region was originally called "Lewisian" by Murchison, from its large development in the Island of Lewis, and I think it would be, for the present at least, an advantage to retain this geographical appellation. At first this "fundamental gneiss" was thought to be a comparatively simple formation, and the general impression probably was that it should be regarded as a metamorphic mass, produced mainly from the alterations of very ancient stratified rocks. Its foliation-planes were believed to be those of original deposit which by terrestrial disturbance had been thrown into numerous plications and corrugated puckerings. But a detailed study of this primeval rock has revealed in it a far more complicated structure. The supposed bedding-planes have been ascertained to have nothing to do with sedimentary stratification, and the gneiss has been resolved into a complex series of eruptive rocks, varying from a highly basic to an acid type, and manifestly belonging to different times of extrusion. With the exception of one district, to which I shall immediately refer, no part of the whole region yet examined has revealed to the rigid scrutiny of my colleagues of the Geological Survey, any trace of rocks which can be regarded as probably of other than igneous origin. It is true that our researches have been hitherto confined to the mainland of Scotland, the large area of the Outer Hebrides, which consists of similar gneisses, remaining to be explored. It is therefore possible that indisputable evidence of an ancient sedimentary series through which the gneiss was originally protruded, may yet be discovered in the unexplored islands. But taking the gneiss as at present known in Sutherland and Rosshire, we find it to be generally coarse in texture, rudely foliated, and passing sometimes into massive types in which foliation is either faintly developed or entirely absent. Much of this gneiss is considerably more basic than the more typical rocks to which the term gneiss was formerly restricted. It consists of plagioclase felspar with pyroxene, hornblende, and magnetite, sometimes with blue opalescent quartz, and sometimes with black mica. These predominant minerals are segregated in different proportions in the different bands, some bands consisting mainly of pyroxene or hornblende, with little or no plagioclase, others chiefly of plagioclase, with small quantities of the ferro-magnesian minerals and quartz, others of plagioclase and quartz, others of magnetite. This separation of mineral constituents can hardly be attributed to mere mechanical deformation. It rather resembles the segregation layers which may be studied in intrusive sills and other deep-seated masses of eruptive material, and which are obviously due to a process of separation that went on while the igneous magma was still in a liquid or viscous condition. At the same time it is manifest that extensive dynamical changes have affected the rocks since the appearance of this original banded structure.
There is further evidence that beside the original eruptive masses, which for want of any means of discriminating their relative dates of protrusion must in the meantime be regarded as belonging to one eruptive period, other portions of igneous material have been subsequently and at successive epochs, after the first mechanical deformations, injected into the body of the original gneiss. These consist of dykes of basalt and dolerite, followed by still more basic peridotites and picrites, and lastly by emanations from a distinctly acid magma in the form of granites. The oldest or doleritic dykes form a wonderful feature in the gneiss, from their abundance, persistence and uniformity of trend in a west-northwest direction. They have no parallel in British Geology until we reach the crowded dykes of older Tertiary time.
Throughout this remarkable complex of eruptive material, though its different portions present many features that may be compared with those of intrusive bosses and sheets belonging to later geological periods, there is no trace of any superficial volcanic manifestation. No tuffs or agglomerates or slaggy lavas have been detected, such as might serve to indicate the ejection of volcanic materials to the surface. All the phenomena of the Lewisian gneiss point to the consolidation of successively protruded portions of eruptive material at some depth within the crust.
Nevertheless it may yet be possible to show that these deep seated masses have been injected into rocks of older date and of sedimentary origin, and that they have communicated with the surface in true volcanic eruptions. I have already alluded to one limited area where various rocks exist, distinctly different from the prevalent types in the Lewisian gneiss. In the area which is traversed by the long valley of Loch Maree in western Rosshire, there occur clay-slates, fine mica schists, graphitic schists, and saccharoid limestones. These rocks remind us of some of the prevalent members of a series of metamorphosed sediments. The minerals enclosed in the marbles are just such as might be expected in the metamorphic aureole of a granite boss, piercing limestone. But the relations of this group of rocks to the ordinary gneiss of the region are not quite so clear as could be desired, though they seem to point to these rocks being surrounded by and enclosed within the gneiss.
The detailed field-work of the officers of the Geological Survey has made known the remarkable amount of mechanical deformation which the various rock-masses composing the Lewisian gneiss have undergone. These rocks have been compressed, crushed, and drawn out, until what were originally massive crystalline protrusions have been converted into perfect schists. The dykes of dolerite have been transformed into hornblende-schists and the granitic pegmatites have been reduced to a kind of powder which has been rolled out so as to simulate the flow-structure of a lava. There is evidence that most, if not all, of this dynamical change was effected long before the deposition of the Torridonian series, for the latter rests in nearly horizontal sheets, with a strong unconformability upon the crushed and sheared gneiss.
Torridon Sandstone. This group of rocks covers only a limited area in the north-west of Scotland, but it must once have spread over a far more extensive region. It reaches a thickness, as I have said, of 8,000 or 10,000 feet, and consists almost wholly of dull, purplish-red sandstones, often pebbly, and bands of conglomerate. Dark grey shales, already alluded to as occurring towards the base of the series, are repeated also in the highest visible portion, and have yielded tracks of what seem to have been annelids and casts of nail-like bodies which may have been organic. I have said that the Torridonian deposits which were classed by Murchison as Cambrian, have been proved by the discovery of the Olenellus zone in an unconformable position above them, to be of pre-Cambrian age. Except along the line of disturbance to which I shall immediately refer, these strata are quite unaltered. Indeed, in general aspect they look as young as the old red sandstones with which Hugh Miller identified them. It is at first hard to believe that such flat undisturbed sandstones are of higher antiquity than the very oldest Palæozoic strata which are so generally plicated and cleaved.
The interval of time between the deposition of the Torridon Sandstone and of the overlying Cambrian formations must have been of enormous duration, for the unconformability is so violent that the lowest Cambrian strata, not only transgressively overspread all the Torridonian horizons, but even lie here and there directly on the old gneiss, the whole of the intervening thick mass of sandstone having been there removed by previous denudation. At Durness, in the north of Sutherland, about 2000 feet of Cambrian (possibly in part Lower Silurian) strata can be traced, the lower portion consisting of quartzites, the central and upper parts of various limestones, sometimes abundantly fossiliferous. Nowhere else in the north of Scotland can so thick a mass of early Palæozoic rocks be seen. Elsewhere the limestones have been in large measure replaced by a complex group of schistose rocks which rest upon the Cambrian strata, and like them dip, generally at gentle angles, towards the east. It was the opinion of Murchison, and was commonly admitted by geologists, that these overlying schists represented a thick group of sediments, which, originally deposited continuously after the limestones, had been subsequently altered into their present condition by regional metamorphism. They were variously named the "Eastern schists," the "younger gneiss," the "gneissose and quartzose flagstones." Nicol, who at first shared the general opinion regarding them, afterwards maintained that they did not belong to a later formation than the limestones, but were really only the old gneiss, brought up again from beneath by enormous dislocations and over-thrusts. We now know from the labors of Professor Lapworth and the officers of the Geological Survey, that Murchison and Nicol had each seized on an essential part of the problem, but that both of them had missed the true solution. Murchison was in error in regarding his younger gneiss as a continuous sequence of altered sedimentary rocks conformably resting on the Cambrian (or to use his terminology, Lower-Silurian) formations. But he sagaciously observed the coincidence of dip and strike between the schists and sedimentary rocks below them and inferred that this coincidence, traceable for many leagues, proved that the metamorphism which had given these schists their structure must have taken place after the deposition of the Durness limestones. Nicol, on the other hand, with great insight recognized that there was no continuous sequence above those limestones, but that masses of the old gneiss had been thrust over them by gigantic faults. But he failed to see that no mere faults would account for the coincidence between the structural lines just referred to in the Cambrian strata, and in the overlying schists, and that the general tectonic structures and lithological characters of the eastern schists differed in many respects from those of the Lewisian gneiss.
The problems in tectonic geology presented by the complicated structures of the northwest of Scotland have been ably worked out by the officers of the Geological Survey, to whose report in the Quarterly Journal of the Geological Society for 1888, I would refer for full details. It has been shown that, besides stupendous dislocations and horizontal displacements, the rocks have been cut into innumerable slices which have been driven over each other from the eastward, while at the same time there has been such a general shearing of the whole region that for many hundreds of square miles the original rock-structures have been entirely effaced, and have been replaced by new divisional planes, which, when they approach the underlying Cambrian strata, are roughly parallel with the bedding planes of these strata.
In this region, therefore, we have striking proofs of a stupendous post-Cambrian regional metamorphism. But there is still much uncertainty regarding the geological age of the rocks which have been affected by it. There can be no doubt that large masses of the old gneiss, torn up from below, have been thrust bodily westward for many miles, and are now seen with their dykes and pegmatites resting on the Durness limestones and quartzites. It is equally certain that in other districts huge slices of the Torridon sandstones have been similarly treated. But where all trace of original structure has disappeared, we have, as yet, no means of definitely determining from what formation the present eastern schists have been produced. The ordinary gneissose and quartzose flagstones do not appear to me to be such rocks as could ever be manufactured by any chemical or mechanical process out of the average type of Lewisian gneiss. I have long held the belief that they were originally sediments, but whether they represent altered Torridon Sandstone, or some clastic formations which may have followed the Durness limestones, but which have been everywhere and entirely metamorphosed, remains for future discovery. For my present purpose, it is sufficient to observe that, in the meantime, as we can not be sure of the origin of most of the rocks, which, between the West Coast and the line of the Great Glen, have been subjected to a gigantic post-Cambrian regional metamorphism, it seems safest to exclude them from an enumeration of the pre-Cambrian rocks of Britain.
Dalradian. East of the line of Great Glen, which cuts the Scottish Highlands in two, another group of crystalline schistose rocks is largely developed. It consists mainly of what were undoubtedly originally sedimentary deposits, though they are now found in the form of quartzites, phyllites, graphitic schists, mica-schists, marbles, and various other foliated masses. With them are associated numerous eruptive rocks, both acid and basic, sometimes still massive and easily recognizable as intrusive, sometimes more or less distinctly foliated and passing into different gneisses, hornblende-schists, chloritic-schists, etc. Though it is not always possible in such a series of metamorphic rocks to be certain of any real chronological order of succession, those of the Highland tracts have now been mapped in detail over so wide an area, that we are probably justified in believing that a definite sequence can be established among them. These masses must be many thousand feet thick. Their succession and association of materials are so unlike those of any of the known older Palæozoic rocks of Britain, that they can hardly be the metamorphosed equivalents of any strata which can be recognized in an unaltered condition in these islands. Some traces of annelid casts have been found in the quartzites, but otherwise the whole series has remained entirely barren of organic remains.
What then is the age of this important series? I must confess that in the meantime I can give no satisfactory answer to this question. I have proposed, for the sake of distinction and convenient reference, to call these rocks "Dalradian." Murchison supposed them to be a continuation of his Durness quartzites, limestones, and "younger gneiss." His belief may still prove to be in some measure well founded. But at present we have no means of deciding whether the quartzites and limestones of the Central Highlands are the more altered equivalents of the undoubtedly Cambrian strata of the north-west. It is possible that in the vast mass of metamorphosed rocks constituting the wide stretch of country from the northern headlands of Aberdeen to the south-western promontories of Argyllshire, there may be portions of the old Lewisian gneiss, tracts of highly altered Torridon sandstone, belts of true counterparts of the Cambrian quartzites and limestones of Durness, and, what should not be forgotten, considerable portions of some later sedimentary series which may have followed these limestones, but which, by the great dislocations already referred to, have disappeared from the north-west of Scotland. We are gradually learning more of these rocks, as the detailed mapping of them by the Geological Survey advances, and when the ground on either side of the Great Glen is surveyed, it may be possible to speak with more certainty regarding their true geological relations.
A glance at a geological map of the British Isles will show that the metamorphic rocks of the south-western Highlands of Scotland are prolonged into the north of Ireland, where they spread over a region many hundred square miles in extent. They retain there the same general character and present the same difficult problems as to their true stratigraphical relations. Quite recently, however, a new light seems to have arisen upon these Irish rocks. My colleagues on the Irish Branch of the Geological Survey have detected several detached areas of coarse gneisses, which in many respects resemble parts of the Lewisian gneiss of north-west Scotland. In some cases these areas lie amidst or close to "Dalradian" rocks, but with that obstinacy, which so tries the patience of the field-geologist, they have persistently refused to disclose their true original position with regard to these. Some fault, thrust-plane, tract of boulder-clay or stretch of bog is sure to intervene along the very junction-line where the desired sections might have been looked for. There can be little doubt that a strong unconformability exists between them. A close examination of the ridge of old gneiss in Tyrone and Fermanagh showed me that though the actual basement-beds of this Dalradian series could not be seen resting on the coarse gneiss, the lithological character, and tectonic arrangement of this series are only explicable on the supposition of a complete discordance between it and the gneiss. As these two groups of rock have never been found in close proximity in Scotland, and as the determination of the true age of the Dalradian series is a question of such great stratigraphical importance in the general mapping of the United Kingdom, I requested Mr. A. McHenry, of the Geological Survey of Ireland, to continue the tracing of the mutual boundaries of the old gneiss of the Ox Mountains and the Dalradian series in County Mayo. He informs me that he has found in that series a conglomerate full of blocks of the old gneiss, and resting in one locality apparently unconformably upon it. If this observation is confirmed it will finally set at rest the relative position of the coarse massive gneiss and some portion, at least, of the Dalradian series. Of course there is no absolute proof that the coarse gneisses of Ireland are really the equivalents of the Lewisian masses which they so closely resemble. But there is a strong presumption in favor of their identity.
In England and Wales many detached areas of rock have been claimed as pre-Cambrian, and successive formations have been classified among them. I have already dealt in part with this question, and without attempting here to review the voluminous literature of the subject, I will content myself with stating briefly what seems to me to have been established on good evidence.
There can not, I think, be now any doubt that small tracts of gneiss, quite comparable in lithological character to portions of the Lewisian rocks of the north-west of Scotland, rise to the surface in a few places in England and Wales. In the heart of Anglesey, for example, a tract of such rocks presents some striking external or scenic resemblance to the characteristic types of ground where the oldest gneiss forms the surface in Scotland and the west of Ireland. In the Malvern Hills another small knob of somewhat similar material is obviously far more ancient than the Cambrian rocks of that locality. There may possibly be still some further exposures of similar rocks in the south of England, as for instance in southern Cornwall. In Anglesey a series of schists, quartzites and limestones has been included by Mr. J. F. Blake with the coarse gneiss above referred to, and a thick higher group of slates in what he terms the "Monian" system. These schists, quartzites and limestones present a close resemblance to the Dalradian series of Scotland and Ireland, and the quartzites, like those of the Highlands, contain worm-burrows. The coarse gneiss, as I have said, may be compared in general character with parts of the Lewisian rocks, so that we seem to have here, as in Ireland, two groups of schistose rocks, and both of these must be much older than the unaltered Cambrian strata which lie above them.
Along the eastern borders of Wales, there is an interrupted ridge of igneous rocks which were originally supposed to have broken through the older Palæozoic formations, but which now, owing mainly to the labors of Dr. Callaway and Professor Lapworth, are shown to be older than the base of the Cambrian system. These rocks consist of spherulitic and perlitic felsites, with volcanic breccias and tuffs. They are undoubtedly older than the Olenellus zone. Though the evidence is not quite satisfactory, they may not impossibly lie at the base of a vast mass of sedimentary rocks forming the ridge of the Longmynd. In that case the whole of the Longmynd succession with the volcanic group at its base must be pre-Cambrian and lie unconformably below the Olenellus zone. Dr. Callaway has proposed the name "Uriconian" for this volcanic group, while the sedimentary series has been termed "Longmyndian." On the supposition that the unconformability is established, there would here be a vast mass of stratified and partly erupted material forming a pre-Cambrian formation. Whether in that case any portion of this English series is the equivalent of the Torridonian rocks of Scotland remains to be determined. The northwestern part of the Longmynd ridge is made of red sandstones and conglomerates, which certainly resemble the Torridonian rocks of Ross and Sutherland.
At the base of the Cambrian rocks in Wales, Dr. Hicks has described a marked volcanic series under the name of "Pebidian," which he claims as pre-Cambrian, alleging that it is separated from the Cambrian system by an unconformability, and a band of conglomerates. I have carefully studied the evidence on this ground, and have come to the conclusion that there is no unconformability at the line in question, but that the ordinary Cambrian strata graduate downwards into the volcanic group and can not be disjoined from it. I therefore regard the so-called "Pebidian" as merely marking the duration of a volcanic period in early Cambrian time.
It will thus be seen that according to my view the unmistakably pre-Cambrian rocks of Britain consist of, first and oldest, the Lewisian gneiss; second, the Torridonian sandstones and conglomerates. The Uriconian and Longmyndian formations may prove to be in part or in whole equivalents of the Torridonian. The Dalradian rocks have not yet had their position determined. They may possibly mark a distinct pre-Cambrian series, but it seems quite as probable that they are only a metamorphic complex in which Archæan, Torridonian and Cambrian, or even Lower Silurian rocks are included.
Sir Archibald Geikie,
Director-General of the Geological Survey of Great Britain and Ireland.
[ARE THERE TRACES OF GLACIAL MAN IN THE TRENTON GRAVELS?]
In a paper published in Science, Nov. 25. 1892, I undertook to study the evidence relating to paleolithic man in the eastern United States from a new point of view,—that furnished by certain recently acquired knowledge of the contents of quarries and shops where modern aboriginal flaked implements were made. It was shown that all rudely flaked forms could be sufficiently accounted for without the necessity of assuming a very rude state of culture, and that any people, paleolithic or neolithic, would in roughing out blades—the principal product of the flaking process—produce precisely these forms and in great numbers as refuse. It further appeared that the finding of these objects in sporadic cases in glacial gravels or in any formation whatsoever, could not be considered as proving or tending to establish the existence of a particular grade of stone-age culture for the region in which the formation occurs, since they may as readily pertain to a neolithic as to a paleolithic status. It was conclusively shown that no worked stone that can with reasonable safety be called an implement has been reported from the gravels, and that it is therefore clearly useless, not to say unscientific, to go on enlarging upon the evidence of an American paleolithic period and multiplying theoretic details of its culture.
I now propose to review briefly the question of the age of our so-called paleolithic implements, the questions of the grade of a given feature of culture and of the age or chronologic place of that culture being very properly treated separately, as they depend for their support upon distinct classes of evidence. During the past summer, 1892, certain important items of new evidence have been discovered bearing upon the question of the occurrence or non-occurrence of rudely flaked stones or of any artificial objects whatsoever in the normal gravels of the Delaware Valley, and it therefore becomes necessary to examine somewhat critically such of the published evidence as seems to be seriously affected by these recent observations.
It may be stated in beginning that no one disputes the glacial age of the Trenton gravels. The question to be discussed is simply this,—is the evidence satisfactory that works of art have been found in these gravels? Nothing else need be asked or answered. I do not take up this subject because I love controversy; disputation is really most distasteful to me. It happens that under the Bureau of Ethnology of the Smithsonian Institution I have been assigned to the work of making a survey of the archeology of the Atlantic coast region in which large areas, especially in states south of Mason and Dixon's line, remained almost untouched by investigators, and two years have been consumed mainly in these southern areas. But there are questions that refuse to be confined to definite geographic limits, and evidence secured in one section is sometimes found to bear so directly and forcibly upon problems pertaining primarily to other sections that the student of these problems must perforce become a free lance, and unhesitatingly enter any province promising results of value, howsoever fully occupied it may be by other investigators. One of the most interesting and important questions growing out of the study of American archeology has, as we have seen, arisen in the Delaware Valley, and the turn taken by some of my work in the south and west is such that I cannot pass this question by without consideration. The necessity of taking up the subject of glacial man became more and more apparent as the years passed on, and people continued to say to me, "You must go to Trenton; we are not satisfied with the present status of the question there; the evidence arrayed in favor of the theory of a paleolithic gravel man needs critical examination."
The difficulty of taking up and re-examining evidence, of which the record only remains, is, however, very great, since in most cases the evidence rests upon or consists of field observations, and these cannot be recalled or repeated, and there is absolutely no means of testing directly the value of what is recorded. One may seek either to verify or to discredit the promulgated theories, but years of search may fail to produce a single new item of evidence bearing decisively upon the subject. It is possible that at one period numerous finds of implements should be reported from certain portions of the gravels, and that afterwards the whole remaining body of these formations should be worked over and searched without securing a trace of art; yet this latter evidence, being negative, need not necessarily be considered sufficient to overturn the original positive evidence if that happens to be of a high class. There is not the least doubt, however, that positive evidence may be so impaired by various defects and inconsistencies, that, unsupported by renewed and well verified observations, it will finally yield to the negative forces; and if the theories of a gravel man in the eastern United States, howsoever fortified by accumulated observations, are not really properly supported in every way, they are bound in time to fall to the ground. All I can reasonably hope to do now is to have the evidence relating to glacial man placed on trial, and so fully examined and cross-examined that those who accept gravel man need not longer do so blindly without knowing that there are two sides to the question, and those who do not accept him may know something of the reasons for the belief that is in them.
The evidence employed to prove the presence of a race of men in the Delaware Valley in glacial times is confined almost wholly to the alleged discovery of rude implements in the glacial gravels. Practically all the evidence has been collected by Dr. C. C. Abbott, and upon his skill as an observer, his faithfulness as a recorder, his correctness of judgment and his integrity of character, the whole matter stands. Many visitors, men of high repute in archeology and geology, have visited the site, but the observations made on such occasions appear not to have been of a nature to be of great value in evidence, the finds being doubtful works of art or not having properly established relationships with the gravels in place. In the discussion of gravel man in eastern America a wide range of objects and phenomena has been considered, but the real evidence, upon which the theory of an ancient race and a peculiar culture must depend, is furnished by a hundred pieces—more or less—of rudely flaked stones said to have come from the gravels in place. And now what can be said with reference to this series of flaked stones further than that they are reported by the collector to have been found in the gravels at definite stated depths? I have elsewhere shown that they are not demonstrably implements in any case, that they are identical in every respect with the quarry-shop rejects of the American Indian, that they do not closely resemble any one of the well established types of European paleolithic implements, and that they are not a sufficient index of a particular stage of culture. I shall now present such reasons as there may be for the belief, held by many, that they were not really found in the undisturbed glacial gravels.
It is generally understood that the earliest reported gravel finds of importance were made on the banks of Assanpink creek within the city limits of Trenton, where the gravels to a thickness of twenty feet or more were exposed in a railway cutting. Later the river bluff near the lower end of the city, where the gravels were exposed to a depth of from twenty-five to forty feet, yielded large numbers. These two sites, so far as I can learn, furnished at least three-fourths of the finds in place. Other specimens were found singly in slight natural exposures, and in excavations for cellars, sewers, etc., at various points within the city limits.
The river bluff was for a considerable period the favorite hunting ground of the searchers for rudely flaked stones, and many specimens were collected. The gravels were exposed in a steep, nearly straight bank, several hundred yards in length, the base of which was washed by the river. There can be no question that Dr. Abbott and others have found shaped objects of various classes upon and in the face of this river bluff, and the visitor to-day, although the bluff is now buried almost completely under city refuse, will hardly fail to find some rudely flaked form in the deeper gullies or upon the narrow river bank or beach at the base. Dr. Abbott explicitly states[1] that he obtained certain of these specimens from the gravel outcrops, and that they were not in talus formations, but in undisturbed deposits. How then is it possible to do otherwise than accept these statements as satisfactory and final?
[1] Abbott, C. C. Primitive Industry, pp. 493-510.
Fig. 1. Sketch map of the Trenton bluff, showing the relation of the sewer trench to the "implement" yielding slope.... a-b section line, [Fig. 2].
Very recently, however, fortunate circumstances have brought the evidence furnished by this site again within our reach, thus enabling us to re-open the discussion under favorable conditions. What I had for some time desired to do in this case was, what I had already done at Piny Branch, D. C., and at Little Falls, Minn., to open a trench into the face of the bluff, and thus secure evidence for or against the theory of a gravel man. This measure was, however, rendered impracticable by the occupation of the bluff margin by a city street; but it happened last summer that the city authorities, desiring to improve the sanitary condition of the city, decided to open a great sewer through this very bluff to get a lower outlet to the river. A trench twelve feet wide and some thirty feet deep, the full depth of the exposed gravels, was carried along the bluff just inside of its margin, opening out into the river at the point where the bluff turns toward the north-east. It was a trenching more complete and more satisfactory than any of which I had ever dreamed. At no point for the entire length of the bluff did the excavation depart more than forty feet from the line of the terrace face—from the upper margin of the slope upon which such plentiful evidence of a supposed gravel man had been obtained. The accompanying map and section, Figs. 1 and 2, will indicate the location of the trench, and show the exact relations of the natural and artificial exposures of the gravels.
Fig. 2. Sections made by the river and by the sewer, the former yielding many "implements," the latter yielding none.
I made several visits to the place, descended frequently into the great cut and examined the gravels and their contents with the utmost care, but without securing a trace of art. Recognizing the vital importance of utilizing to the fullest extent this opportunity of testing the art-bearing nature of the gravels at this point, I resolved to undertake a systematic study of the subject. Summoning my assistant, Mr. William Dinwiddie, from his field of operations in the South, I had him spend upwards of a month at the great trench, faithfully watching the gravels as they were exposed. Mr. Dinwiddie had worked three years under my personal direction, and had helped open upwards of twenty trenches through similar gravel deposits, and was therefore well qualified for the work. Prof. W. J. McGee, Prof. R. D. Salisbury, Dr. Stewart Culin and Dr. Abbott also visited the place one or more times each. Relics of art were found upon the surface and in such portions of the talus as happened to be exposed, but nothing whatever was found in the gravels in place, and the search was closed when it became fully apparent that the case was hopeless.
It may be claimed that the conditions under which gravels are exposed in trenching as it progresses, are not as favorable for the collection of enclosed relics as where exposed by natural processes of weathering. This is true in a certain measure, as specimens may be obscured by the damp clinging sand which forms the matrix of the gravels. This, however, would interfere but little with the discovery of large flaked stones, such as we were led to expect in this place, and this slight disadvantage in detecting shaped pieces in fresh exposures is more than over-balanced by the treachery of weathered surfaces which often give to intrusive objects the appearance of original inclusion. The opportunity for studying the gravels in all their phases of bedding, composition and contents, was really excellent, and no one could watch the constantly renewed exposures hour after hour for a month without forming a most decided notion as to the implement bearing qualities of the formation. Not the trace of a flaked stone, or of a flake or artificial fragment of any kind was found, and we closed the work with the firm conviction that the gravels exposed by this trench were absolutely barren of art. But Dr. Abbott claims to have found numerous implements in the bluff face a few feet away and in the same gravels. If this is true, the conditions of glacial occupation of this site must have been indeed remarkable. It is implied that during the whole period occupied by the melting of the ice sheet within the drainage of the Delaware valley the hypothetical rude race lived on a particular line or zone afterwards exposed by the river to the depth of 30 feet, leaving his strange "tools" there by the hundreds, while another line or zone, not more than forty feet away at most, exposed to the same depth by an artificial trench, was so avoided by him that it does not furnish the least memento of his presence. One vertical slice of the gravels twelve feet thick does not yield even a broken stone, while another slice not probably one-half as thick, cut obliquely through the gravels near by, has furnished subject-matter for numerous books and substantiation for a brace of theories. That no natural line of demarcation between the two section lines is possible, is shown by the fact that the formations are continuous, and that the deposits indicate a constant shifting of lines and areas of accumulation; thus it was impossible for any race to dwell continuously upon any spot, line or plane. This is well shown in the section, [Fig. 3], which gives the relations of the art-producing section of Dr. Abbott to the non-art-producing section of the sewer. The gravels were laid down entirely irrespective of subsequent cutting, natural or artificial; yet we are expected to believe that a so-called gravel man could have resorted for a thousand years to the space a, leaving his half shaped or incipient tools at all stages of the gravel building from base to top, failing entirely to visit a neighboring space b, or to leave there a single flake to reward the most faithful search. It is much easier to believe that one man should err than that a guileless race should thus conspire with a heartless nature to accomplish such extraordinary results. The easier explanation of the whole matter is that the objects found by Dr. Abbott were not really in the gravels, but that they are Indian shop-refuse settled into the old talus deposits of the bluff, and that his eager eyes, blinded by a prevailing belief in a paleolithic man for all the world alike, failed to observe with their wonted keenness and power.
Fig. 3. a, Reputed "implement" producing zone of the river front. b, Barren zone of sewer.
But this case does not stand alone. The first discoveries of supposed gravel implements are said to have been made when the Pennsylvania Railway opened a road bed through the creek terrace on the site of the present station. At first numerous specimens of rudely flaked stones were reported, and the locality became widely known to archeologists, but the implement bearing portions of the gravels—and this is a most significant fact—were limited in extent, and the deposit was soon completely removed, the horizontal extension containing nothing. At present there are excellent exposures of the full thickness of the gravels at this point, but the most diligent search is vain, the only result of days of examination being a deep conviction that these gravels are and always were wholly barren of art.
It thus appears that here as well as upon the river front, the works of art were confined to local deposits, limited horizontally but not vertically, and a strong presumption is created that the finds were confined to redistributed gravels settled upon the terrace face in the form of talus. Dr. Abbott states that "at that point where I gathered the majority of specimens there is a want of stratification."[2] It is well known that such rearranged deposits are often difficult to distinguish from the original gravels. In trenching an implement producing terrace at Washington—where the conditions were probably quite similar to those at the Trenton railroad station—I passed through eighty feet of redistributed talus gravels before encountering the gravels in place, and so deceptively were portions of these deposits re-set that experts in gravel phenomena were unable to decide whether they were or were not portions of the original formation (cretaceous). The question was finally settled by the discovery of artificially shaped stones in and beneath the deposits.
[2] Abbott, C. C. 10th Annual Report of the Peabody Museum, p. 41.
Again, an implement bearing deposit of gravel was recently discovered by the late Miss F. E. Babbitt at Little Falls, Minnesota, and sufficient (a very little) digging was done to satisfy the discoverer, and all paleolithic archeologists as well, that the objects were really imbedded in the glacial gravels. In the summer of 1892 I visited the place and carried a trench twenty feet horizontally into the terrace face on the "implement bed" level before encountering the gravels in place. The talus deposits were several feet thick, and were of such a nature that their true character could not be determined without careful and extensive trenching. The whole talus deposit was here well stocked with Indian quartz quarry-shop rejects, which were as usual of paleolithic types, and it was but natural that Miss Babbitt's conclusions, although based as they necessarily were upon inexpert observations, backed by such well known "types" of "implements" should be unhesitatingly accepted by believers.
The occurrence of these telling examples of the deceptive appearance of re-set gravels would seem to justify and emphasize the conviction created by a critical examination of the two leading so-called paleolithic sites at Trenton, that Dr. Abbott, notwithstanding his asseverations to the contrary, has been deceived. Very strong support, it seems to me, is given to this conclusion by the recently published opinion of the late Dr. H. Carvill Lewis, a glacialist familiar with the Trenton region, and with the work of Dr. Abbott at the period of his paleolithic castle building. Dr. Lewis is reported to have maintained before an open meeting of the Academy of Science in Philadelphia "that what Dr. Abbott believed to be undisturbed layers (of gravel) were those of an ancient talus."[3] This remark may refer to both the main sites—the one at the railroad station and the other at the river front—or possibly only to the former. I have also heard it stated that that eminent scholar, Dr. Leidy, who must have had ample opportunities of forming correct opinions upon the subject, held pretty much the same views of Dr. Abbott's finds.
[3] Brinton, D. G., Science, Oct. 28, p. 249.
Fig. 4. A freshly formed gravel bluff.
Fig. 5. Early stage of talus formation.
Fig. 6. An ancient talus.
To make the above criticism entirely clear, a few words of explanation of talus phenomena may be added. As a river cuts its channel deeper and deeper into deposits of gravel a section is gradually exposed, but the gravels break down readily under atmospheric influences and the exposed face does not retain a high angle. The upper part crumbles and descends toward the base, there to rest against the slope or to be carried away by the stream. A supposititious case will be convenient for illustration. A gravel terrace twenty feet in height is encroached upon by the river at high water and undermined, and the face breaks down vertically, leaving an exposure as illustrated in [Fig. 4]. In a very short time the upper portions become loosened and fall below, giving a steep slope as seen in [Fig. 5]. The process goes on with gradually decreasing rapidity, and if the river does not again encroach seriously, a practically stable slope is reached, as shown in [Fig. 6]. Such a talus may be hundreds or even thousands of years old, but there is rarely any means of determining its exact age. If the gravels are homogeneous in character, the talus will simulate their normal condition so completely that the distinction cannot be made out in ordinary gullies or by unsystematic digging. If the gravels contain varied strata the talus will be composite, and will be more readily distinguished from at least portions of the material in place.
Now it is important to observe what may be the possible art contents of such a talus as that shown in [Fig. 6]. It may contain all objects of art originally included in that portion of the gravels represented by a, b, c, together with all articles that happened to be upon the surface b, c, beside such objects as may have accumulated from dwelling or shop work upon its own surface, after the slope became sufficiently reduced to be occupied for these purposes. A talus is therefore liable to contain, and in the utmost confusion, relics of all periods of occupation, supposing always that there were such periods, from the beginning of the formation of the gravel deposits down to the present moment. As a rule such a talus, if art-containing, will have a large percentage of shop and quarry-shop refuse, for the reason that the exposed gravels, and the banks and beds of rivers cutting them, furnish, as a rule, a good deal of the raw material utilized by workers in stone, and the shops in which the work was done are usually located upon the slopes and outer margins of the terraces. Although there is the possibility of very considerable age for these talus deposits, it is unlikely that any of them date back as far as the close of the glacial epoch or at all near it, for rivers change back and forth constantly, undermining first one bank and then the other, so that a very large percentage of our talus deposits have been formed well within the historic period.
At Trenton the constantly exposed gravel banks afforded considerable argillite in bowlders, fragments and heavy masses, as well as some other flakable stones of inferior quality little used, and it is inevitable that the Indian who dwelt upon the shores of the river should have sought the workable pieces along the bluff, leaving the refuse everywhere; and it is a necessary consequence that the terrace margin, the bluff face, and the talus deposits, places little fitted for habitation, should for long distances contain no trace of any art shapes save such as pertain to manufacture. Thus are fully and satisfactorily accounted for all the turtle backs and other rude forms that our paleolith hunters have been so assiduously gathering. Nothing can be more fully apparent than that no other race than the Indian in his historic character and condition need be conjured up to reasonably account for every phase and every article of the recovered art. Mistaken interpretations of the nature of shop rejects, and the common association of these objects with redistributed gravels, are probably accountable for the many misconceptions that have arisen. Talus deposits form exceedingly treacherous records for the would-be chronologist. They are the reef upon which more than one paleolithic adventurer has been wrecked.
Relics of art attributed to gravel man have been collected, so far as I can gather from museum labels and from incidental references in various publications, from a number of sites aside from the two already referred to. These are scattered over the city, and the finds were made mostly in exposures of the gravels that remained visible for a short time only, as in street and cellar excavations and well pits. These reported finds can never be brought within the range of re-examination, and the searcher after unimpeachable testimony must content himself with placing them in the doubtful column on general principles. Urban districts are so subject to disturbance through cutting down of hills, filling in of depressions, grading of streets, digging of foundations, cellars, sewers, wells and graves that no man can, from a limited exposure such as those producing the reported tools necessarily were, speak with certainty of the undisturbed nature of the deposits penetrated. It is doubtful if any one is justified in publishing such observations at all without serious query. Such testimony is liable to fall of its own inherent weakness, being absolutely valueless if unsupported by collateral evidence of real weight. It can only be made permanently available to science by the discovery of something unusual or unique with which to couple it, something decidedly un-Indian in character or type, as for example the two skulls now in the Peabody Museum. These objects and the antler knife-handle exhibited with them may be alluded to as the only finds so far made at Trenton, having of themselves the least potentiality as proof and these skulls and this knife-handle must yet be subjected to the rigid examination made necessary by the importance of the conclusions to be based upon them.
Something may now be said concerning the art remains upon which this discussion hinges, and upon which conclusions of the greatest importance to anthropology are supposed to depend. Let us pass over all that has been said with regard to their manner of occurrence and association with the gravels and ask them simply what story they tell of themselves. Does this story, so far as we are able clearly to read it, speak of a great antiquity and a peculiar culture, or does it hint rather at vital weaknesses in the position taken by the advocates of these ideas? We shall see. The history of the utilization of rudely flaked stones in the attempt to establish a gravel man in America has never been written, but as read between the lines of paleolithic literature, it runs about as follows: The theory of a very rude and ancient people, having a unique culture and certain peculiar art limitations, was developed in Europe many years ago in a manner well known and often rehearsed. This people was associated with the ice age in Europe, and this epoch, with its moraines and till and sedimented gravels, was found to have been repeated in America. It was the most natural thing possible that these discoveries should carry with them the suggestion that man may have existed here as in Europe during that epoch, and that his culture was of closely corresponding grade. These were legitimate inferences and warranted the instituting of careful researches, but it was a dangerous suggestion to put into the minds of enthusiastic novices with fertile brains and ready pens. The idea was hardly transplanted to American soil before finds began to be made. The so-called "types" of European paleoliths suggested the lines upon which finds here should be made, and everything in the way of flaked stones connected directly or indirectly with the glacial gravels which had not yet been fully credited to and absorbed by the inconvenient Indian, was seized upon as representing the ancient time and its hypothetic people and culture. In the early days of the investigation the various rude forms of flaked stones, resulting from failures in manufacture, had not been studied, and were shrouded in convenient mystery, and they thus became the foundation of the new archeologic dynasty in America, the dynasty of the turtle-back. Dr. Abbott states in his first work[4] that these rude "implements" are not especially characteristic of any one locality, but seem to be scattered uniformly over the state. Specimens of every type, he says, are "found upon the surface, and are plowed up every spring and autumn; but this in no way militates against the opinion that these ruder forms are far older than the well-chipped jasper and beautifully-polished porphyry stone-work."[5] At that stage of the investigation it was not at all necessary that a specimen should come from the gravels in place or from any given depth, since the "type" was supposed to be easily recognized and was a sufficient means of settling the question of age.
[4] Abbott, C. C. The Stone Age in N. J., Sm. Rep. 1875, p. 247.
[5] Ibid, p. 252.
Rude "implements" were called for and they were found. The only requirements were that they should not be of well-known Indian types, that they should be rude and have some sort of resemblance to what were known as paleolithic implements abroad. Since most of these so-called gravel implements of Europe are also doubtless the rejects of manufacture resemblances were readily found. The early attempts to utilize these rejects in support of the theory, and make them masquerade creditably as "implements" with specialized features and self-evident adaptation to definite ice-age uses, now appear decidedly amusing. Gradually, however, the lines have been drawn upon this early license, and it is to-day well understood by all careful students, that since the rude forms are so often repeated in modern neolithic refuse, the only reliable test of a gravel "implement" is its occurrence in the gravels in place. That a particular "implement," said to have been obtained from the gravels, is of "paleolithic type," does not in the least strengthen its claims to being a bona fide gravel implement; nor does its easy assignment to a "type" give any additional value to the collector's claim that the gravels said to contain it are implement bearing. The very names, "rude implement," "paleolithic implement," etc., carry with them a certain amount of mysterious suggestion; one thinks of unique, significant shapes and of strange, archaic uses. At their mere mention, the great ice sheet looms up with startling realism, and the reindeer and the mighty mammoth appear upon the scene. The reader of our paleolithic literature is led to feel that these antiquated objects carry volumes of history in their worn and weather-beaten faces, but this is all the figment of fertile brains. These objects have without exception the appearance of the most commonplace every-day rejects of manufacture without specialization and without hidden meaning. They tell of themselves no story whatsoever, save that of the oft-repeated failure of the aboriginal blade maker in his struggle with refractory stones. This will be shown with greater clearness farther on.
But the scheme does not end with the repetition of a European state of affairs. Our gravel archeologists have not been content to adopt that feature of the foreign scheme which utterly destroys the paleolithic race before a higher culture is brought upon the scene. It was thought to improve upon the borrowed plan by allowing for a gradual development upward from the paleolithic stage, represented exclusively by a class of meaningless bits of flaked stone, through a period less rude, characterized by productions so far advanced as to be assigned to a definite use. These latter productions consist mainly of rather large and often rude blades, sometimes plain, but generally notched or modified at the broader end as if to be set in a handle, or attached to a spear or arrow shaft. These were assigned to post-glacial times in such a way as to bridge or partly bridge the great space between the glacial epoch and the present. They were separated arbitrarily from the body of the collections of the region, and referred to as probably the work of an Eskimo race. This arrangement produced a pleasing symmetry and completeness, and brought the history of man down to the beginning of the Indian epoch, which is represented by all of those forms of art with which the red man is historically associated.
Three principal periods are thus thought to be represented by the finds at Trenton; and in the arrangement of the collections these grand divisions are illustrated by three great groups of relics, which are looked upon by the founders of the scheme as an epitome of native American art and culture. By others this grouping is looked upon as purely empirical, as an arbitrary separation of the normal art remains of the historic Indian, not suggested by anything in the nature or condition of the objects, nor in the manner of their discovery.
The "Eskimo" feature of the scheme requires a more detailed examination than can be given it here. It may be stated, however, that the separation of the so-called Eskimo spear points, or whatever they may be, from the great body of associated articles of flaked stone, appears to be a highly arbitrary proceeding. That they were extensively made by the Indians is proved by the occurrence of refuse resulting from their manufacture on modern shop sites, and that they were used by the Indian, is equally apparent from their common occurrence on modern dwelling sites. The exceptionally large size of the argellite points is readily accounted for by the nature of the material. It was the only stone of the region well adapted to the manufacture of long blades or projectile points. Jasper, quartz and flint have such minute cleavage that, save in rare cases, small implements only could be made from them. Their peculiar manner of occurrence, described at so much length by Dr. Abbott,[6] has been given undue consideration and weight. The phenomena observed may all be accounted for as a result of the vicissitudes of aboriginal life and occupation within the last few hundred years as fully and satisfactorily as by jumping thousands of years backward into the unknown.
[6] Abbott, C. C. Popular Science Monthly, Dec., 1889.
Whatsoever real support there may be for the "Eskimo" theory, either in the published or the unpublished evidence, it is apparent that under the present system of solitary and inexpert research, the scientific world will gain little that it can utilize without distrust and danger. Whatsoever may be the final outcome—which outcome is bound to be the truth—it is clear that there is little in the present evidence to warrant the separation of a "paleolithic" and an "Eskimo" period of art from that of the Indian.
That the art remains of the Trenton region are essentially a unit, having no natural separation into time, culture or stock groups, is easily susceptible of demonstration. I have already presented strong reasons for concluding that all the finds upon the Trenton sites are from the surface or from recent deposits, and that all may reasonably be assigned to the Indian. A find has recently been made which furnishes full and decisive evidence upon this point. At Point Pleasant, on the Delaware, some twenty-five miles above Trenton, there are outcrops of argillite, and here have been discovered recently the shop sites upon which this stone was worked. There are two features of these shops to which the closest attention must be given. The first is that they are manifestly modern; they are situated on the present flood plain of the Delaware, and but a few feet above average water level, the glacial terrace here being some forty or fifty feet in height. These shops, therefore, represent the most modern phases of aboriginal industry, and may have been occupied at the coming of William Penn. The second point is that every type of flaked argillite found in the Trenton region, associated with the gravels or otherwise, is found on this site. It was to a certain extent a quarry site, for the great masses of argillite brought down by the floods were here broken up and removed from the river banks or bed. It was a shop site, for here the articles, mainly blades, were roughed out, and it was also a dwelling place—a village site—where all the specialized forms of flaked stones made from the blades were prepared for use. Here are found great numbers of the rude failures, duplicating every feature of the mysterious "paleolith" with which our museums are stocked, and exhibiting the same masterly quitting at just the point "where no further shaping was possible."[7] Here we see the same boldly manipulated "cutting edge," the "flat bottom" and "high peak," and the same mysteriously weathered and disintegrated surfaces, so skillfully made, by a nice balancing of accidents,[8] to tell the story of chronologic sequence in deposition.
[7] Abbott, C. C. Smithsonian Report, 1875, p. 248.
[8] Ibid. Primitive Industry, p. 487.
Beside the failures, we have here, as on other quarry shop sites, the evidence of more advanced work, the wide, thick, defective blades, and many of the long, thin blades broken at or near the finishing point. Here, too, just back of the roughing-out shops, are the dwelling sites from which many specialized forms are obtained. The "Eskimo" type is fully represented as well as the ordinary spear point, the arrow point, and the perforator of our Indian. There is not a type of flaked argillite known in the Delaware valley that may not be duplicated here on this modern Indian site, and this has been known by local archeologists for years. Why so little has been said about the matter is thus explained. Dr. Abbott, in 1890, discovering this site, and finding "typical paleolithic implements" (the ordinary ruder forms of rejects) among the refuse, was so entirely at a loss to explain the occurrence that he felt compelled to again "take up the examination of the gravel deposits of the valley of the Delaware" with the hope of "finally solving the problem."[9] The true conditions would have been at once apparent to any one not utterly blinded by the prevailing misconceptions.
[9] Abbott, C. C. Annual Report of the Curator of the Museum of American Archeology, University of Pennsylvania. No. 1, p. 7.
The entire simplicity of the archeologic conditions in the Delaware valley may be further illustrated. Had William Penn paused in his arduous traffic with the tawny Delawares, and glanced out with far-sighted eyes from beneath the pendant branches of the great elm at Shackamaxon, he might have beheld an uncouth savage laboriously fabricating rude ice age tools, making the clumsy turtle-back, shaping the mysterious paleolith, thus taking that first and most interesting theoretical step in human art and history. Had he looked again a few moments later he might have beheld the same tawny individual deeply absorbed in the task of trimming a long rude spear point of "Eskimo" type from the refractory argillite. If he had again paused when another handful of baubles had been judiciously exchanged, he would have seen the familiar redskin carefully finishing his arrow points and fitting them to their shafts preparatory to a hunting and fishing cruise on the placid Delaware. Thus in a brief space of time Penn might have gleaned the story of the ages—the history of the turtle-back, the long spear point and their allies—as in a single sheaf. But the opportunity was wasted, and the heaps of flinty refuse left upon the river bank by the workmen were the only record left of the nature of the work of that day. Two hundred years of aboriginal misfortune and Quaker inattention and neglect have resulted in so mixing up the simple evidence of a day's work, that it has taken twenty-five years to collect the scattered fragments, to sift, separate and classify them, and to assign them to theoretic places in a scheme of culture evolution that spans ten thousand years.
Yet is there really nothing in it all, in the theories, the observations, the collections and the books? Do I speak too positively in condemnation of the results of years of earnest investigation? Perhaps so, but the voluminous testimony is so overloaded with inaccuracies, the relics of unscientific method and misleading hypotheses, that every item must be sharply questioned; and the conclusions reached so far overstep the limits warranted by the evidence, that heroic measures alone can be effectual in determining their exact value. If, as many believe, vital errors have been embodied in the evidence presented by the advocates of the theory, it is impossible to state the case too strongly. Error once fully absorbed into the literature of science has many advantages over the tardy truth; it is strongly fortified and must be attacked and exposed without fear or favor. Truth involved with it cannot permanently suffer. If the twin theories of a gravel and a paleolithic man in eastern America are to be assailed as unsound or as not properly supported, it should be done now while the originators and upholders are alive and alert to sustain their positions or to yield to the advances of truth. I do not wish to wrongly characterize or to unduly minimize the evidence brought to bear in favor of these theories. I do intend, however, to assist the world so far as possible in securing an exact estimate of all that has been said and done, and all that is to be done.
In a previous article I have examined the evidence relating to paleolithic art in the eastern United States, and have indicated its utter inadequacy and unreliability. In this paper the testimony relating to the occurrence of gravel art, in the locality most fully relied upon by advocates of the theory, has been partially reviewed and subjected to the strong light of recent observations. It is found that the whole fabric, so imposing in books and museums, shrinks away surprisingly as it is approached. The evidence furnished by the bluff face and by the railway cutting, the two leading sites, is fatally weakened by the practical demonstration of the fact that the gravels proper are at these points barren of art remains. In endeavoring to naturalize an immigrant hypothesis, our gravel searchers, unacquainted with the true nature of the objects collected and discussed, and little skilled in the observation of the phenomena by means of which all questions of age must be determined, have undoubtedly made grievous mistakes and have thus misled an expectant and credulous public.
The articles themselves, the so-called gravel finds, when closely studied are found to tell their own story much more fully and accurately than it has heretofore been read by students of archeology. This story is that the art of the Delaware valley is to all intents and purposes a unit, that there is nothing unique or especially primitive or ancient and nothing un-Indian in it all. All forms are found on demonstrably recent sites of manufacture. The rude forms assigned by some to glacial times are all apparently "wasters" of Indian manufacture. The large blades of "Eskimo" type are only the larger blades, knives and spear points of the Indian, separated arbitrarily from the body of the art-remains to subserve the ends of a theory, certain obscure phenomena of occurrence having been found to give color to the proceeding. To place any part of this art, rude or elaborate, permanently in any other than the ordinary Indian category will take stronger proofs than have yet been developed in the region itself.
The question asked in the beginning, "Are there traces of glacial man in the Trenton gravels?" if not answered decisively in the negative, stands little chance, considering present evidence, of being answered in the affirmative. In view of the fact that numerous observations of apparent value have been made in other sections, there is yet sufficient reason for letting the query stand, and we may continue to cherish the hope that possibly by renewed effort and improved methods of investigation, something may yet be found in the Trenton gravels clearly demonstrative of the fascinating belief in a great antiquity for the human race in America.
The evidence upon which paleolithic man in America depends is so intangible that, unsupported by supposed analogies with European conditions and phenomena, and by the suggestions of an ideal scheme of culture progress, it would vanish in thin air; and if the theory of a glacial man can summon to its aid no better testimony than that furnished by the examples examined in this paper, the whole scheme, so elaborately mounted and so confidently proclaimed, is in imminent danger of early collapse.
W. H. Holmes.
[GEOLOGY AS A PART OF A COLLEGE CURRICULUM.]
The demand for scientific studies as a part of the college curriculum is felt by all those who have to do with the provision of higher instruction for American youth. The reasons for this may be various, but a fundamental reason is found in the tendency among the American people in particular, and in this age in general, toward practicality in all things. Applied to education this practicality asks for a training which shall have a direct bearing upon the business of life to be followed immediately after the training period is ended. It means a differentiation of subjects and specialization in methods to adjust the education to the different functions which the students taking it are preparing for. It calls for a professional education for those who expect to become lawyers, doctors, ministers, or teachers,—a technical education for those who are to engage in the arts of the mechanical or civil engineer, or of the architect. It results not only in the establishment of colleges and universities devoted to this kind of education, but it affects the methods of the high schools and academies, and is felt down to primary schools, and on the other hand the older institutions founded on a different plan are adapted to the popular demand by the addition to the regular studies of "electives," chosen not always for their value or disciplinary studies, but because of the practical applicability of the information to be derived from them, to the business of the student.
Without discussing the relative merits of the two ideas of education, the chief contrast between them may be found in the character of the results sought. The knowledge of things and their uses is of chief importance in the practical education; the knowledge of ideas and skill in their use is the aim of the liberal education. Geology is one of the sciences which most men will at once classify as among the practical sciences. It deals with matters of practical importance to everybody. Coal, iron, the metals, silver, gold, tin, lead, building stone, sand, clay, petroleum, and natural gas, and all geological products are essential materials of modern civilization, and a knowledge of them and of their modes and places of occurrence is one of the requisites of an education, either from the practical or the liberal point of view. So too the dynamics of atmospheric and hydraulic erosion, the agency of rivers and oceans in destruction, removal and reconstruction of geological formations have their eminently practical bearings upon the various arts of engineering. While the practical value of geology is thus evident and undisputed, it is not on this account that its importance as a part of a college course of education is urged. As a practical study geology becomes the centre of a group of studies requiring years for mastery. Chemistry and physics are primarily essential to a full understanding of the most common of geological problems. And to use geological facts and phenomena, an acquaintance with the complex methods of engineering, civil and mechanical, which again call for a thorough mastery of mathematics, is necessary. Mineralogy and petrography, metallurgy and mining engineering have each reached a stage of development entitling them to the rank of separate sciences, but the practical training of the geologist should include them all. When we add the biological sciences connected with historical geology, paleontology, zoölogy and botany, with all the laboratory and field work required for their proper study, we have a group of affiliated branches of learning requiring four or five years of continuous study after the student has learned how to study. It is plain therefore that only a specialist, one who is willing to neglect other studies, or who has previously had a liberal training, can perfect himself on the practical side in the science of geology.
But irrespective of its practical uses, as a means of training and supplementary to the ordinary studies of a college curriculum, geology is one of the most useful of the sciences of observation. It is in providing that particular training to which President Eliot has recently called attention in the Forum (Dec., 1892, Wherein Popular Education Has Failed), that geology can be used to such advantage. Speaking particularly of the lower education, President Eliot says it is "the judgment and reasoning powers" that particularly require attention. Their systematic development is to be attained in the four directions of "observing accurately, recording correctly, comparing, grouping and inferring justly, and expressing cogently the results of these mental operations." (p. 421.) The attainment of these ends is one of the purposes of liberal education, whether it be in the primary school or in the university. And geology, or any other science, is of value in a college course in proportion to its fitness for the exercise and development of these functions of the student. Geology may be taught without regard to these ends, and then it is valuable from the practical point of view, but when we examine it in respect of its availability as a disciplinary study we find it offering particular attractions.
Using the distinction between theory and practice, which is as old as Aristotle, geology in its theoretical aspect is more easily comprehended than is the theoretical aspect of most of the modern sciences. This arises first from the fact that the facts and phenomena are of a simple and grand nature, making it possible for the teacher to direct certain attention to the specific facts under consideration. The water of the rivers, the mud by the road side, the rocks and sands on the shore are familiar objects to all, and it is a simple matter to call attention by ordinary language to the specific facts regarding them, which, analyzed out, are to form the basis of exact ideas and scientific definition and classification. Geology is the one science among the natural sciences which may begin with the common language of the pupil, and by means of such language alone may build up ideas of precise phenomena in scientific terms. Physiography or physical geography surpasses geology proper in this particular, as the admirable work of Professor Davis is showing, and on this account it is the best introduction to geology. But the very largeness and indefiniteness of the facts are in the way of the use of physical geography for the exercise of the finer and more exact functions of observation. The disciplinary value of classics and mathematics is to a considerable extent derived from this quality, the precision with which the words or figures kindle like ideas. So long as the object of the training is to teach the knowledge of ideas and how to use them, classics and mathematics are the simplest and purest means of developing a liberal education. The addition of sciences to the college course is not because of the usefulness of the knowledge of things thus to be gained, but because the language of the sciences is essential to call forth the observation and the exercise of the accompanying mental operations.
When it comes to dealing with the ideas associated with particular sense-observation, where form or motion can not be expressed in simple mathematical terms, language can not communicate a new idea or kindle it in another mind with precision. It is necessary by some means to recall or to present the object itself to the student. In the teaching of science this point is of great importance, and much of the unsatisfactoriness of science-teaching is doubtless due to failure to note it. No circumlocution of words can arouse in another or communicate to him the idea appropriate to a sensation he has never felt. The blind man whose eyes are opened sees men as trees walking.
In the use of science for elementary training (and the training is elementary until the student is capable of investigating and interpreting the facts and phenomena of a science directly) that science is the better which deals with objects which are simple, common and easily observed. Such is geology in some of its aspects. Every time the student walks in the country he sees the facts discussed in the text-book or by his teacher; and from attention to those with which he is already familiar he can be readily led to observe and give attention to others and to analyze those already in his mind by properly directed questions.
In the field of geology are found the ready means for the exercise and development of observation and thought. The learner begins with ideas which every intelligent mind associates with the objects described or named, and by degrees the marks of his knowledge are increased, the relations of things are grasped, and the content of his ideas associated with the language of his science is enlarged. In the process of learning the science he has been building up his stock of knowledge of facts and phenomena, but, of more importance than that, he has learned the method of observing and of scientific thinking. He has had training in the methods of reducing the hard facts of nature to the laws of thought and practice, he has seen the method by which theoretical order is made out of the interminable confusion and complexity of natural things.
Beside this primary reason for the use of geology as a disciplinary science-study, there is a second reason arising from the symbolic nature of a large group of its facts. This aspect of the science is best seen in the historical and stratigraphical parts of geology, in which fossils are the chief data for study. The interpretation of a fossil into a species of organism, having its definite place in the elaborate classification of the zoölogist, or as an indicator of the time and place and mode of formation of the strata in which it is buried, is, to be sure, a most intricate and, at first thought it would seem, an unattractive process. But no more so, I would say, than the interpretation of a series of Greek characters. The interpretation of the Greek reveals to us the richest results of human thought and most perfect laws of human speech, and we find therefore in the analysis required the most perfect discipline of the powers of speech and language. The fossil too holds, ready to be revealed, the story of the history of the world and the laws of the evolution of the organic life of the globe, and records an inexhaustible wealth of information regarding the laws of nature. But as an instrument of intellectual discipline its great merit lies in its symbolic nature. It is this symbolic character of the classical languages and of the mathematics which fits them to be universal means of liberal training. The symbolic nature of the fossil fits it to become the exponent of training in the pure science of nature. The fossil is a mark which stands for something, and thus, in the nature of things, it asks for interpretation. As a symbol it stimulates minute and accurate observation, and kindles close and exhaustive thought; as a symbol it leaves us the ideas it has engendered after it is lost to memory as an observation. Thus the value of its study does not depend upon the retention in the memory of the facts brought before the mind, but in the training of the mental processes required in its interpretation. The study of this branch of geology exercises and develops all the faculties which are specially exercised in any scientific investigation.
Another aspect in which it is an ideal means for such training comes from the fact that it is equally valuable at every stage of progress of the student. When first examined it means nothing to him. He knows nothing of organism, of strata, of geological time. The fossil gains meaning only as he is able to put meaning into it. The student must ask questions, and as step by step he answers his questions by more minute and wider examination, the fossil holds a fuller interpretation. His studies lead him to investigation of the whole field of nature, the rocks, the formation of deposits, the action of the elements, the conditions of life, the forms of organism, their functions and habits, the laws of growth, their adaptation to environment, the changes of events in time, the efforts of association and struggle for life, the principles of evolution and development—the migration and origin and extinction of organisms on the globe. Nothing in nature is without interest to him. Further than this the amount of good he gains is not measured by the number of fossils he studies, but by the wideness of his research. A handful of fossils from some one fossiliferous ledge may be the text for a year's study, and the methods acquired in the study may be the nucleus of a life's work. In this department of geology the possibilities for new discoveries, new developments of science are almost endless. As a single author thoroughly read develops a wealth of knowledge of the laws of language and thought, so geology may be studied by the use of a limited set of its phenomena and become the introduction to the exhaustive study of natural science.
Another advantage attaching to geology as a science-study for the college curriculum, arises from the fact that it may be pursued deeply without the elaborate aid to the senses required in other sciences for making minute record or measurement of facts or phenomena. As in language and mathematics, it is essential to acquire a familiarity with the grammar, the dictionary and the symbols, formulas and rules of their usage before the finer training in the use of thought begins, so the vocabulary and the definitions of a science must be acquired before much use can be made of the higher discipline to be derived from scientific study. In language study this higher training comes from practice in making the minute analysis, in detecting the fine shades of meaning expressed in the literature itself. So it is important in selecting a science to be used as a disciplinary study that the facts and laws of nature with which it is concerned should be capable of clear and precise definition, and, moreover, that it should furnish a field for the study of the minute and intricate relationship existing between the different facts which are to be attained by personal inspection of the objects themselves. In most of the sciences this deeper exercise of scientific thought requires for its successful pursuit artificial aids to the common senses of observation. Chemistry must have its purified acids and reagents, test tubes, and delicate scales for measurement of weight and volume. Mineralogy must have its chemical analyses, or optical measurements so fine that microscopes of highest power are essential tools for the investigation. Physics must have the most delicate measurements of time and space and weight. Botany, for the earlier stages of study, is fully equal to geology in these respects, but its scope is much less general. Zoölogy requires dissections calling for skill in manipulation, and in other respects is ill adapted to general classes. But precision in the intellectual processes of observation and reasoning can be cultivated in the use of geological facts to their highest and widest perfection, with scarcely any aids to the normal faculties of observation. A couple of hammers, a pocket lens, a chisel and a few pointed steel tools for revealing fossils, a tape line, compass and clinometer are the few equipments that will enable the geologist to carry his investigations to almost any degree of thoroughness.
What has already been said applies to the study of the pure science of geology either in the field or in the laboratory. There is still another use to which this, as other sciences, may be put in disciplining the college student in directions not provided for by literary or mathematical studies,—the study of man as an investigator. In the pursuit of the study of geology, the first instruction must be received in didactic form, but after the text-book and lecture stage is passed, or while it is under way consultation of the literature of the sciences is appropriate. In the use of scientific literature the critical judgment is brought under training, and the varying interpretations of well known phenomena by expert scientists suggest the prominent part which the notions already in the mind play in the interpretation of the external facts observed. The experienced geologist will recall many cases of honest report of impossible facts by men who are unable to distinguish between what they saw and the false interpretations they made of these observations. One man will report that a live toad jumped out of the middle of a solid piece of coal, when it was heated in the stove; another will swear that he saw a fossil shark's tooth taken out of a ledge of Trenton limestone. It is evident that our memory of observation is not the revival of the object producing the sensation, but of the idea we framed of the sensation at the time. The study of original descriptions of objects of nature reveals the fact that the describer uses the ideas he already has in his mind as he does the standard foot-rule in his hand for measuring that which he describes, and it is by the study of scientific literature and the comparison of views of many scientists that this highest discipline of the observational faculties is attained—the power to determine the personal equation of error for the observer, and thus see through his descriptions a truer representation of the facts than the observer himself saw. Geological literature is admirably adapted for this higher discipline, and in no field of science (I think not in astronomy itself), has wider and more comprehensive thought been applied than in geology. While other branches of science have been developed and become more narrow and special in their treatment of the facts concerned, geology still stands as the most comprehensive of all the sciences of nature.
H. S. Williams.
Yale College, November 30, 1892.
[THE NATURE OF THE ENGLACIAL DRIFT OF THE MISSISSIPPI BASIN.]
It is of some importance, both to the practical work of the field and the theoretical deductions of the study, to determine the nature and amount of the drift that was carried forward in the body of the ancient continental glaciers, and brought out on their terminal slopes and at length deposited at their frontal edges, and to distinguish it from that which was pushed or dragged or rolled along at the bottom of the ice.[10] It may be helpful to indulge in a speculative discussion at the outset to prepare the way for the specific evidence and the inferences to which it leads.
[10] Debris, which may be imbedded in the basal layer of the ice during some part of its transportation, but which is brought down to the bottom and subjected to basal action in the latter part of its course, and ultimately becomes a part of the basal deposit, is not here included in the englacial drift.
Whenever a prominence of rock is overridden and enveloped by a glacier of the free-moving continental type, one of two things takes place; either that part of the ice which passes over the summit of the prominence flows down its lee slope, carrying whatever debris it dislodges down to the rear base, and thence onward along the bottom of the ice, or else the currents which pass on either side of the prominence close in behind it before the corresponding current which passes over the summit reaches the point of their junction, in which case the summit current is forced to pass off more nearly horizontally into the body of the ice, carrying with it whatsoever debris it has dislodged from the summit of the prominence and embodied within its base. The law of the phenomena appears to be that whenever the height of the prominence is less than one-half the base, measured transversely to the movement of the ice, the summit current will follow down the lee slope; but whenever the height of the prominence is more than one-half the transverse base, the lateral currents will close in on the lee, and the summit current will flow off into the body of the ice. This simple law is, however, subject to very considerable modifications from several different sources which may be grouped under (1) differences in the friction arising from basal contact, and (2) differences of internal friction and mobility. The lateral currents will expose more surface to the sides and base of the hill and the adjoining plain, and will be more subject to conflicting currents, while, on the other hand, being deeper currents, they will presumably be more fluent. These and other qualifying conditions will go far to vitiate the application of the law, but its statement may have some value as representing a general conception of the phenomena. When the height of the prominence becomes great relative to the total thickness of the ice, the fluency of the summit current may be much reduced relative to that of the central parts of the lateral currents. When the prominence reaches the surface, blocks dislodged from it are borne away on the surface of the glacier, and constitute superglacial drift. Blocks dislodged from near the summit, but below the surface of the ice, are presumably carried onward in the upper zone of the glacier; while other blocks detached at various but sufficient heights on the side of the prominence are doubtless borne around into the lee and carried forward in the same vertical plane as the summit stream, so that there comes to be a vertical zone set with boulders moving on from the lee side of the nunatak.
Lofty ledges or plateaus, with vertical or undercut faces, furnish similar means for the lodgment of debris within the body of the ice.
In these and doubtless in other ways it appears that there came to be lodged directly within the body of the Pleistocene glaciers at some considerable distances above their bases, blocks derived from rock prominences that rose with sufficient steepness above the general surface of the country over which the ice passed. The lodgment of debris on the lateral borders of glaciers is neglected here because it has little or no applicability to the phenomena of the upper Mississippi basin. It is also doubtful whether any prominences protruded through the ice except near the thin edge, when advancing and retreating, and these are too inconsiderable to merit attention.
It is obvious, upon consideration, that blocks detached from summits or from the sharp angles of out-jutting ledges or plateaus might suffer some glacial abrasion in the process of their dislodgment and transposition along the crest or projecting angle, but that in general such abrasion would be small, and, in most cases, nearly or quite absent. The debris so incorporated in the body of the ice would be, for the most part, angular, and, as it was brought forward in the ice, it would probably suffer very little abrasion. If it continued to move forward in the plane in which it started, descending only so much as the bottom wastage of the ice required, it would be brought out to the terminal slope of the ice sheet by virtue of the melting away of the ice above, and thence it would be carried on down the terminal slope as superglacial debris, and dropped at the frontal edge. If this be the true and full history, there would be no commingling of this englacial matter with the subglacial debris. It is evident, that the englacial matter brought forward from the crest of one prominence would be intermingled with that brought forward from other prominences lying in a line with it, or lying so near it that the lateral spreading of the debris would lead to commingling. It is also clear that variations in the direction of currents would tend to the same result, so that englacial matter from different prominences of the same general region might be commingled. So also englacial material, by crevassing and by the descent of streams from the surface to the base, would be carried down to the bottom and mingled with the subglacial debris. So also blocks broken away from the base of the prominence which yielded the englacial erratics might be moved forward along the bottom parallel with the englacial material above, and lodged at any point along the line. It is therefore to be expected that the basal deposits will contain the same rock species as the englacial, but if there be no process by which the basal material is carried upward the reverse will not be the case, and there will be a clear distinction between the englacial deposit and the subglacial deposit, in composition as well as physical state.
Not a few glacialists, however, advocate in somewhat differing forms and phases the doctrine that basal material is carried upward into the body of the glacier and at length reaches the surface, and that at the extremity of the ice this is commingled with any erratics that may be englacial or superglacial by original derivation. This doctrine appears to have had its origin in the endeavor to explain the very common fact that glacial drift has been carried from lower to higher altitudes. Erratics are often found lodged several hundred feet higher than the outcrop from which they were derived. It has never seemed to me, however, that this phenomenon necessarily was different in kind from that which takes place in the bottom of every stream; at least I have not come in contact with any instances that seemed to require a different explanation, except those connected with kames and eskers that require a special explanation in any case. We are so accustomed to view streams from above, and so accustomed to study the extinct glaciers from the bottom, that we are liable to overlook the community of some of the simpler processes involved alike in both phenomena. The dictum that water never runs up hill is measurably true of the surface currents of the ice as well as water, but it altogether fails when applied to the basal currents of either. It is probable that there is no natural stream of any length in which, at some part of its course, basal debris is not carried from lower to higher altitudes and lodged there. If the bed of any stream were made dry and the debris in it critically examined, it would be found that at numerous points the silts or sands or gravels had been carried from the bottom of some basin in its bed to the higher rim or bar or reef that bordered it on the downstream side. So I conceive that, on a grander scale, the natural result of the flow of the basal ice of a continental glacier over the inequalities of the country was the lifting of material from some of the lower horizons and its lodgment on the crests of ridges or the slopes or summits of mountains that lay athwart its course.
So again, it is certain that a considerable part of the peripheral drainage of glaciers takes place through tunnels beneath the ice. It is reasonable to suppose that during the winter season, when the drainage is slack, these tunnels tend to collapse in greater or less degree, under the continued pressure of the ice and the "fattening" of the glacier, so that in the early part of the next melting season the contracted tunnels may be over-flooded by glacial waters. To the extent that these tunnels become incompetent the water would become ponded back in the crevasses and moulins by which the surface-water gains access to them. They thus come to have something of the force of water flowing in tubes, and may be presumed to be capable of forcing rounded material to some considerable height, and of carrying ice-imbedded boulders to any point reached by the stream. These tunnels probably undulate with the bottom, and lodgment along them takes place wherever enlargement permits.
Without, therefore, appealing to any upward cross currents within the ice itself, it is possible to explain the transportation of the drift from lower to higher altitudes. I have never seen phenomena of this kind that seemed to call for any other explanation than these. I am not prepared to say that there are no such phenomena. One of the purposes of this article will have been accomplished, if it shall call forth a critical statement of phenomena that require the assumption of internal upward movements of the ice to account for them, and of the criteria which distinguish such phenomena from those that may be referred to upward basal movements such as are common to all streams or to the exceptionally conditioned subglacial streams. That there are upward internal movements in most streams is as much beyond question as the existence of upward basal currents in rivers and glaciers, but they are dependent chiefly upon the velocity of the current and the irregularity of the bottom. Theoretically, as I understand, a stream moving in a straight course on a perfectly smooth bottom would not develop an upward cross current. Each lower layer would move slower than that above it by reason of basal friction, but they would move on in parallel lines. But if irregularity of bottom be introduced the parallelism is obviously destroyed, and if the velocity be high so that the momentum of the particles becomes great relative to their cohesion, irregular internal movements will result, and these will often be of a rotary nature in vertical planes bringing the basal parts of the fluid to the surface or the reverse. For this reason rapid streams abound in rotary currents, while slow streams do not.
Now it is quite obvious that a stream of water moving at a rate of three or four feet per day, or even fifty or sixty feet per day, would not develop perceptible upward currents, and certainly would not lift the lightest silt from its bottom. I do not think there are any theoretical grounds for believing that internal glacial currents are developed, which flow from base to surface, carrying bottom debris to the top.
One of the most remarkable expressions of the drift phenomena of the Upper Mississippi region consists of belts of boulders stretching for great distances over the face of the country, and disposing themselves in great loops after the fashion of the terminal moraines of the region with which they are intimately connected. Besides this, there are numerous patches of boulders of more or less irregular form and uncertain relations. The whole of these have not been studied in detail, but a sufficient portion of them have received careful examination to justify the drawing of certain conclusions from them. Those which have been most studied lie in Ohio, Indiana, Illinois, Michigan, Wisconsin, Iowa and Dakota. Those of the first three States have been most carefully traced and their constitution is such as to give them the greatest discriminative value. To these our discussion will be limited chiefly.[11]
[11] Parts of these tracts were long since described by Bradley of the Illinois Survey. (Geol. Surv. Ill., Vol. IV. p. 227). Collet of the Indiana Survey (An. Rep. 1875, p. 404) and Orton and Hussey of the Ohio Survey (Geol. Surv. Ohio, Vol. III., pp. 412, 414 and 475). The relationship of these tracts to morainic lines and to each other I worked out some years since (Third An. Rep. U. S. G. S. pp. 331, 332, 334) but I owe many details and some important additions to my associate, Mr. Leverett.
Emerging from the dunes at a point north of the Iroquois river in Jasper county, northwestern Indiana, a well characterized belt of surface boulders stretches westward to the State line, just beyond which it curves about to the south and then to the east, and re-enters Indiana a little south of the northwest corner of Benton county. It soon turns abruptly to the south and reaches the Wabash river near the centre of Warren county. The immediate valley of the Wabash is thickly strewn with boulders from the point where the belt reaches it to the vicinity of West Point on the western line of Tippecanoe county. The uplands, however, do not give any clear indication of the continuity of the belt, and the connection is not altogether certain. There is an inner well-marked belt that branches away from this in the central part of Benton county and runs southeasterly into the northwestern quarter of Tippecanoe county, beyond which only scattered boulders occur, which leaves its precise connections also in doubt. But starting from West Point, which is less than a dozen miles from the point where the two belts cease to be traceable with certainty, a well-defined belt, one or two miles wide, runs southeasterly across the southwestern corner of Tippecanoe county and the northeastern quarter of Montgomery county to the vicinity of Darlington, beyond which its connection is again obscure, although boulders occur frequently between this point and the northwestern corner of Brown county, where boulders are very abundant. So also, patches of exceptionally abundant boulders occur in the west central part of Clinton county. These may be entitled to be regarded as a connecting link between the train which enters northwestern Tippecanoe county and that of northwestern Boone county, as scattered boulders of the surface type, but of not very exceptionally frequent occurrence, lie between them. However this may be, a belt of much more than usually frequent surface boulders stretches southeasterly to the vicinity of Indianapolis, and probably connects with a very well-marked belt lying near the south line of the southeast quarter of Marion county and in the northeastern part of Johnson county. There is also a well-defined tract in southeastern Hendricks county, running east and west, without evident connection with the foregoing tracts, though it may be the equivalent of the Darlington belt. There is also a somewhat unusual aggregation in the form of irregular belts in southeastern Johnson county, in the vicinity of Nineveh, and in southern Shelby county. The belt south of Indianapolis is probably to be correlated by scattered boulders only slightly more abundant than those of the adjacent region, but of the surface type, stretching northeasterly to near the center of the west half of Henry county, where a well-marked belt again sets in. From this point the tract runs northeasterly nearly to the north limit of the county, where it turns easterly and runs in the vicinity of the line between Randolph and Wayne counties to near the Ohio line, where it curves to the southeast entering Ohio near the northwest corner of Preble county. In its southeasterly course across that county it is phenomenally developed as has been well shown by the descriptions of Professor Orton. Soon after entering Montgomery county it curves about to a northeasterly course, and crossing the great Miami river, a few miles above Dayton, holds its northeast course across the southeastern part of Miami county, the northwestern part of Champaign county, and thence on to about the center of Logan county, where it curves about and runs in a direction a little east of south to near the southeast corner of Champaign county, beyond which it ceases to be a specially notable phenomenon.
In the region between the Wabash and Kankakee rivers, in northern Indiana, there are numerous tracts of irregular form over which surface boulders in phenomenal abundance are scattered. These are particularly noticeable in southern Jasper county; in the vicinity of Wolcott, Monon and Chalmers in White county; near Star City in Pulaski county; in the southeastern corner of Stark county, and very generally along the great interlobate moraines, lying parallel with the Eel river, and some others of the Saginaw glacial lobe. These are so associated with the inter-tangled morainic phenomena of that region as not to admit of convenient and brief description in their genetic relationships.
The well-defined tracts have a most significant distribution. The first part described is associated with the terminal moraine that marked the margin of a lobe of ice that moved westward along the axis of the Iroquois basin to a point a few miles beyond the Indiana-Illinois line. The portion that runs southward to the Wabash is associated with the moraine that follows the same course, and runs at right angles over the older moraines of the Lake Michigan lobe. The tract in Tippecanoe and Montgomery counties, that in south Marion county, and that in Henry and Randolph counties, in the eastern part of the state, are associated with the terminal moraines that form a broad loop with the West White river basin lying in its axis. In western Ohio the belt is intimately associated with a moraine that bordered the Miami lobe of the ice sheet, and the south-trending portion in eastern Logan and Champaign counties lies on the western margin of the Scioto lobe.
The relationship of these tracts to terminal moraines is very clear and specific. They constitute marginal phenomena of the ancient ice sheet. Their distribution completely excludes their reference to floating ice, for they not only undulate over the surface utterly negligent of any horizontal distribution, but they are disposed in loops in crossing the basins of the region, and the convexities of these loops are turned down stream. These basins for the most part open out in southerly or westerly directions which makes it improbable that ice-bearing bodies of water occupied them. But if this were not fatal, certainly the fact that the convexities of the boulder belts are turned down stream and cross the centers of the basins is precisely contrary to the distribution they must have assumed if they were due to floating ice in bodies of water occupying the basins. I hold it, therefore, to be beyond rational question that these tracts were deposited as we find them by the margins of the glacial lobes that invaded the region.
If these boulder belts were of the same nature as the average boulders of the till-sheets beneath them, then the simple fact of unusual aggregation might be plausibly referred to the accidents of gathering and deposition. But they are very clearly distinguished from the average boulders of the till by several characteristics.
1. They are superficial. Sometimes they rest completely on the surface, sometimes they are very slightly imbedded, sometimes half buried, sometimes they protrude but a slight portion, and sometimes they are entirely concealed, but lie immediately at the surface. In all cases the aggregation is distinctly superficial. Where they are buried, the burying material is usually of different texture and composition from the subjacent till, and appears to be distinct in origin from it. The superficiality of the tract is very obvious almost everywhere, and is especially so in regions where the subjacent till is of the pebble-clay rather than boulder-clay order, for the comparative absence of boulders below emphasizes the contrast. Throughout most of the region the subjacent till is not of a very bouldery type, so that the distinction is generally a marked one.
2. The boulders of the belts are almost without exception derivatives from the crystalline terranes of Canada. Those of the great tract especially under consideration were derived from the typical Huronian rocks of the region north of Lake Huron, and from granitic and gneissoid rocks referable to the Laurentian series of the same region. These last, however, cannot be sharply distinguished from the granitic rocks derived from other parts of the Laurentian terrane. The Huronian rocks are very easily identified because of the peculiarities of some of the species. Among these the one most conspicuously characterized is a quartz-and-jasper conglomerate. The matrix is usually a whitish quartzite. This is studded with pebbles of typical red jasper and of duller rocks of jasperoid nature, which grade thence into typical quartzite pebbles. With these are mingled crystalline pebbles of other varieties. Another peculiar erratic comes from the "slate conglomerate" of Logan. It consists of a slaty matrix through which are scattered rather distantly pebbles of granitic, quartzitic and other crystalline rocks. This is one of the forms of the "basal conglomerate" of Irving. Other varieties of this "basal conglomerate" are present. In addition to these very peculiar rocks, a quartzite of a very light greenish semi-translucent hue has a wide distribution along the tract. It is readily distinguishable from the numerous other quartzites of the drift of the interior. Some years since, on returning from my first field examination of a portion of this belt, I sent a typical series of chips from the characteristic erratics to Professor Irving, who had recently returned from the study of the original Huronian region. He returned a suite of chippings that matched them perfectly throughout, all of which were taken in situ in the region north of Lake Huron.
Among the boulders of the belt are occasionally found specimens of impure limestone or of limy sandstone that might perhaps be referred doubtfully to some member of the paleozoic series; but on the other hand, might with equal or greater probability perhaps be referred to the similar rocks of the Huronian series. These are quite rare, never forming, so far as my observations go, as much as one per cent. of the series. In the several definite enumerations made to determine the percentage of the doubtful specimens, the result never exceeded a fraction of one per cent. In the most extensive enumeration the result was about one-half of one per cent. Aside from these doubtful specimens there are practically no boulders in the belts that can be referred to any of the paleozoic rocks that intervene in the 500 miles between the parent series north of Lake Huron and the tract over which the boulders are now strewn. Occasionally there may be seen erratics from the paleozoic series at or near the surface, but they are not usually so disposed on the surface as to appear to be true members of the superficial boulder tract. There is, therefore, the amplest ground for the assertion that these boulder tracts are of distant derivation, and that they are essentially uncommingled with derivatives from the intermediate region.
3. The boulders of this series are much more angular than those of the typical till sheets. Some of them, indeed, are rounded, but the rounding is generally of the type which boulders derived by surface degradation and exfoliation present. They rarely have the forms that are distinctively glacial. Quite a large percentage are notably angular, and have neither suffered glacial rounding nor spherical exfoliation. Some few are glacially worn and scratched, but the percentage of these is small.
The tracts therefore present these four salient characteristics: (1) the boulders are derived from distant crystalline terranes (400 to 500 miles) and are essentially uncommingled with rock from the intervening paleozoic terranes; (2) they are essentially superficial, and the associated earthy material has a texture differing from that of the subglacial tills; (3) they are notably angular and free from glacial abrasion, except in minor degree; (4) the tracts are so associated with terminal moraines and so related to the topography of the region, that there is no rational ground for doubt that the boulders were borne to their present places by the glaciers that produced the correlative moraines.
In contrast to these superficial boulder formations, the till sheets below are made up of a very large percentage of glacial clay whose constitution shows that it was produced in part by the grinding down of the paleozoic series. In this are imbedded boulders and pebbles that were derived from the paleozoic series as indicated by their petrological character, and, in many instances, demonstrated by contained fossils. While a small part of the boulders contained in the till are angular or but slightly worn, the larger part are blunted, bruised, scratched and polished by typical glacial action. This obvious grinding of the boulders, taken in connection with the clay product resulting from the grinding, affords a clear demonstration that the deposit was produced at the base of the ice by its pushing, dragging, rolling action.
The two formations, therefore, stand in sharp contrast; the one indicating the passive transporting action of the ice in bearing from their distant homes north of the lakes the crystalline boulders and dropping them quietly on the surface, the other indicating the active dynamic function of the ice in rubbing, bruising and scoring the material at its base. The one seems to me a clear instance of englacial and superglacial transportation; the other an equally clear example of subglacial push, drag and kneading.
Now if it were the habit of an ice-sheet of this kind to carry material from its bottom to the surface by internal movement, it would seem that the distance of 400 to 500 miles which intervened between the source of the crystallines and the place of their deposit would have furnished ample opportunity for its exercise, and that there would have been commingled with the englacial and superglacial material many derivatives from the intermediate region, and these derivatives should have borne the characteristic markings received by them while at the base of the ice. The very conspicuous absence of such commingling, and the absence or phenomenal rarity of anything that even looks like such a commingling, appears to me to testify in quite unmistakable terms to the distinctness of the methods of transportation. In view of the great territory over which this particular belt is spread, and the greater territory which is embraced in the other tracts not here specially considered, there is left little ground for doubt that this distinctness of englacial from basal transportation was a prevailing fact and not an exceptional one. This is supported by concurrent evidence derived from the territory west of Lake Michigan. This territory unfortunately does not bear erratics that have equally distinct characteristics, but, so far as my observation goes, the phenomena are alike throughout. I am therefore brought to the conclusion that, in the interior at least, there was no habitual lifting of boulders from the base of the ice sheets to the surface, nor any habitual commingling of basal with englacial and superglacial material, except, of course, as it took place by virtue of the falling of the latter through crevasses to the base, and by mechanical intermixture of the two at the edge of the ice.
The amount of englacial till under this view is little more than that which was lodged in the body of the ice in its passage over the knobs and ridges of the hilly and semi-mountainous regions of the north. To this is perhaps to be added occasional derivatives from the more abrupt prominences of the paleozoic region and the superficial dust blown upon the ice from the surrounding land, which was probably the chief source of the silty material intermingled with the superficial boulders. The total amount is thus quite small, though important in its significance.
The eskers and kames of the region are made up of derivatives from the basal material as shown by (1) the local origin of the material in large part, (2) the mechanical origin of the sands and silts, (3) the not infrequent glacial markings of the pebbles and boulders, and (4) the disturbed stratification of the beds.[12] If I am correct in respect to the kind and amount of the englacial and superglacial material, it is obvious that eskers and kames, such as are found in the interior, could not be derived from englacial or superglacial sources. The term englacial as here used does not include such materials as may be lodged in the basal stratum of the ice and brought down to the actual bottom by basal melting.
[12] See "Hillocks of Angular Gravel and Disturbed Stratification," Am. Jour. Sci. Vol. XXVII., May 1884, pp. 378-390.
The conclusions drawn from the phenomena of the plains of the interior are not necessarily applicable to more hilly or mountainous regions.
T. C. Chamberlin.
[Studies for Students.]
DISTINCT GLACIAL EPOCHS, AND THE CRITERIA FOR THEIR RECOGNITION.[13]
[13] Read before the American Geological Society at Ottawa, Dec., 1892.