Relics of Primeval Life: Beginning of Life in the Dawn of Geological Time

by Sir John William Dawson

III

PRE-CAMBRIAN LIFE

H

HAVING traced the chain of life through the long geological ages, from the present day back to the Cambrian Period, we may now take our stand on the fauna of the lowest Cambrian or Olenellus Zone, as a platform whence we may dive into still deeper abysses of past time. Here, however, we seem to have arrived at a limit beyond which few remains of living things have yet been discovered, though there still remain pre-Cambrian deposits of vast thickness and occupying large areas of our continents. These pre-Cambrian formations are as yet among those least known to geologists. The absence of fossils, the disturbances and alterations which the rocks themselves have undergone, and which make their relative ages and arrangement difficult to unravel, have acted as deterrents to amateur geologists, and have to some extent baffled the efforts of official explorers. In addition to this, workers in different regions have adopted different methods of arrangement and nomenclature; and in a very recent address, the Director-General of the Geological Survey of Great Britain expresses his inability to satisfy himself of the equivalency of the different pre-Cambrian groups on the opposite sides of the Atlantic, and in consequence prefers to retain for those of Britain merely local names.

On the other hand, those who hold the modern theories of gradual evolution repudiate the idea that the Lower Cambrian fauna can be primitive, and demand a vast series of changes in previous time to prepare the way for it. In any case this comparatively unexplored portion of geological time holds out the inducement of mystery and the possibility of great discoveries to the hardy adventurers who may enter into it. It must now be our effort to explore this dim and mysterious dawn of life, and to ascertain what forms, if any, are visible amid its fogs and mists.

The Kewenian or Etcheminian.

In certain basal Cambrian or infra-Cambrian beds, found by Matthew in Southern New Brunswick, by Walcott in Colorado, and by Scandinavian and English geologists in their respective countries, we find a few remains referred to Algæ, or seaweeds; small tests or shells of Protozoa; burrows and trails similar to those of modern sea-worms; a few bivalve shells allied to modern Lingulæ, but presenting some remarkable generalized characters; some bivalve and shrimp-like Crustaceans, spicules of sponges, and large laminated forms (Cryptozoon) similar to those already referred to as occurring in the Upper Cambrian; also certain mysterious markings that are supposed to have been produced by the arms or tentacles of free-swimming animals of various kinds. In these lower beds the Trilobites have nearly or quite disappeared, being represented only by doubtful fragments. The beds of rock, originally sandy or muddy sediments, contain fossils very sparingly, and only in certain layers separated by great thicknesses of barren material, as if earthy matters were being deposited very rapidly, or as if animal life was rare on the sea-bottom except at intervals. It has, however, been suggested as possible[6] that much of the marine population in those early times consisted of pelagic or swimming animals destitute of any hard parts that could be preserved. In addition to biological arguments in favour of this view, there is the fact that some of the beds are stained with carbonaceous or coaly matter, as if the sediment had been mixed with decomposed remains of plants or animals retaining no determinate forms. Future discoveries may increase our knowledge of the life of this period preceding the Cambrian, but it is evident that so far as these rocks have been examined, they indicate a great step downward in regard to the variety and complexity of marine life.

[6] By Prof. Brookes, of Johns Hopkins University.

Still we must bear in mind that in later periods there have been times of rapid deposition, in which, in certain localities at least, great thicknesses of rock with few organic remains were formed. We have instances of this in the later Cambrian, in the Ordovician, and still later in the Permian and Trias. Thus in the beds immediately underlying the lowest Cambrian we may be passing through a tract of comparative barrenness to find more fertile ground below.

It is also to be observed that there is evidence of disturbance occurring in the interval between the lowest Cambrian and the highest pre-Cambrian, which may involve the lapse of much time not recorded in the localities hitherto explored, but of which monuments may be found elsewhere.

We may now, taking some North American localities as our best available guides, inquire as to the nature and contents of the beds next below the Lower Cambrian.

Fig. 10.—Section at Hanford Brook. (After Matthew.)
Showing St. John group resting on Etcheminian, and this on Coldbrook (Huronian).

In Southern New Brunswick, Matthew indicated, several years ago, the occurrence of certain conglomerates and sandy and slaty beds over the rocks, mostly of igneous origin, constituting a great thickness of beds under the Cambrian, and known locally as the "Coldbrook" series, which is probably equivalent to the Huronian of Northern and Western Canada, to be noticed later. These beds were at first regarded as an upper member of the Huronian, but subsequently it was thought better to unite them with the overlying Cambrian as basal Cambrian. The fact that these problematical beds were ascertained to be unconformable to the Cambrian, and the peculiarity of their fossils, led to their being constituted a separate group under the name Etcheminian, which seems to represent a time and conditions introductory to the Cambrian ([Fig. 10]). The fossils in these beds are few and hard to find. Matthew has kindly furnished me with the following list.[7] The Trilobites are conspicuous by their absence. Sea-worms have left burrows, trails, and casts, which probably represent several species ([Fig. 11]). A single little shell (Volborthella) is supposed to be a precursor of the straight chambered shells allied to the modern nautilus, which become so large and numerous in succeeding periods. There are a few univalve shell-fishes allied to modern sea-snails, a brachiopod of the antique genus Obolus, some fragments supposed to represent Cystideans, a rudimentary type of the stalked sea-stars so abundant later, spicules of sponges and minute Protozoa, with shells not unlike those of their modern successors. This meagre list sums up the forms of life known in the Etcheminian of this district, one in which the Cambrian beds exhibit the rich and varied fauna of Trilobites and other animals described and figured by Matthew in several successive volumes of the "Transactions of the Royal Society of Canada" ([Fig. 12]).

[7] "Transactions Royal Society of Canada," vol. vii.

Fig. 11.—Trails of Worms of two types (Psammchnites and Planilites).

Beds in Newfoundland (the Signal Hill and Random Sound series), underlying the Lower Cambrian, have afforded to Murray and Billings some well-characterized worm-castings of spiral form, and a few problematical forms known as Aspidella, which may be Crustaceans or Mollusks allied to the limpets ([Fig. 13]).

Fig. 12.—Group of pre-Cambrian (Etcheminian) Animals from the Etcheminian. (After Matthew.)
The name "Etcheminian" is derived from that of an ancient Indian tribe of New Brunswick.
(a) Volborthella, supposed to be a Cephalopod shell. (b) Pelagiella. (c) Orthotheca, supposed to be Pteropods. (d) Primitia, an Ostracod Crustacean, (e) Obolus, a Brachiopod shell. (f) Platysolenites, probably fragment of a Cystidean. (g) Globigerinæ, casts of Foraminiferal shells, Etcheminian, New Brunswick.

Fig. 13.—Arenicolites (Spiroscolex) spirales (Billings) and Aspidella tenanovica (Billings), Signal Hill Series, Newfoundland.

Fig. 14.—Fragment of Cryptozoon, Grand Cañon, Arizona.
Photograph from a specimen presented by Dr Walcott to the Peter Redpath Museum.

In a thick series of pre-Cambrian beds in the Colorado Cañon in the Western United States, Walcott has found a small roundish shell of uncertain affinities,[8] a species of Hyolithes, probably a swimming sea-snail or Pteropod, a small fragment which may possibly have belonged to a Trilobite, and some laminated forms which, if organic, are related to the Cryptozoon already mentioned ([Fig. 14]).

[8] Discinoid or Patelloid.

The Kewenian series of Lake Superior has yielded no fossils, but the pipestone beds of Minnesota, supposed to be about the same age, have afforded a small bivalve shell allied to Lingula;[9] and the black shales of the head of Lake Superior contain some impressions supposed to be trails of animals.[10]

[9] Winchell.

[10] Selwyn and Matthew.

It has been a question whether the beds above referred to should be regarded as a downward continuation of the Cambrian, or as the upper part of an older system. Matthew, whose opinion on such a subject is of the highest authority, regards them as a distinct system, but as belonging, with the Cambrian, to the great Palæozoic Period. Van Hise, and some other United States authorities, would separate them even from the Palæozoic, and unite them with the underlying Huronian, as representing a "Proterozoic" or "Algonkian" Period. This is merely a matter of classification, necessarily more or less arbitrary; but I believe the facts to be stated subsequently show that it will be best to unite the Etcheminian and its equivalents with the Palæozoic, and to place the groups lower than this in one great division, equivalent to Palæozoic, and for which many years ago I proposed the name "Eozoic," or that of the Dawn of Life.

Having thus hastily glanced at the slender fauna of the rocks immediately below the Cambrian, we may now proceed to inquire a little more in detail into its true value and import as leading toward the beginning of life. I have already referred to the apparently sudden drop in the number of groups and of species below the base of the Cambrian, and have hinted that this may be an effect of temporary local conditions of deposit or of defective information. Another fact that strikes us is the diverse and miscellaneous character of the fossils that remain to us; and this would suggest that we are either dealing with a mere handful picked at random, as it were out of a richer fauna, or that in the beginning of things the gaps and missing links between different forms of life were even more pronounced than at present. This, however, would be likely to occur if the plan of creation was to represent at first different types, with few forms in each; to produce, in short, a sort of type collection representing the whole range of organization by a few characteristic things rather than to give a complete series, with all the intermediate connections. Such a mode of introduction of life is not à priori improbable, however at variance with some prevalent hypotheses.

Beginning with the higher Invertebrates, we must not conclude that we have altogether lost the Trilobites. The fragments referred to this group may represent at least a few species, and it would be very interesting to know more of these as to their relations to their successors, and whether they are tending to lower or more embryonic forms. The bivalve Crustaceans (Ostracods) may be regarded as inferior in rank to the Trilobites, but are still very complex, and specialized animals and a specimen silicified in such a manner as to show the interior organs testified that, as far back as the Carboniferous at least, these creatures were as highly organized as at present,[11] while their generally larger size in the earlier formations tends to show that they have rather been degenerating in the lapse of geological time.

[11] Palæocypris Edwardsi, Brougniart, Coal Formation of St. Etienne, France.

In regard to the Sea-worms, the burrows, castings, and trails found in the pre-Cambrian beds are scarcely, if at all, different from those now seen on sandy and muddy shores, and would seem to indicate that these highly organized and very sensitive and active creatures swarmed in the muddy bottom of the pre-Cambrian Sea, and lived in the same way as at present. It is impossible, however, to know anything of the internal structures of these creatures, but the marks left by their bristle-bearing feet seem to indicate that some of them at least belong to the higher group of Sea-centipedes, creatures rivalling the Crustaceans in complexity of organization, and near to them in plan of structure, though at present usually widely separated from them in current systems of classification. In the Ordovician system, next above the Cambrian, Hinde has found many curiously formed jaws of animals of this kind, which show at least that their alimentary arrangements were similar to those now in force. If any of the problematical "Conodonts" discovered by Pander in the Cambrian of Russia belonged to marine worms, this inference would be extended back to the Lower Cambrian, so that if the evidence of structure anywhere remains we may hope to find that the pre-Cambrian worms were not inferior to their more modern successors, perhaps even that in this early period, when they probably played a more important part in nature, they were of higher organization than in later times.

The evidence as to pre-Cambrian mollusks, so far as it goes, is even more curious. The little shell called Volborthella, so far as can be judged from its form and internal structure, is a miniature representative of these straight Nautili, the Orthoceratites of the Ordovician and later Palæozoic rocks; and no one doubts that these latter belong to the highest class of the Mollusks, a class approaching in the development of nerve system and sensory organs to the Vertebrates themselves. This tiny member of the great class of Cuttle-fishes may perhaps have been more nearly allied to the modern Spirula than to the Nautilus. In any case, if, as seems altogether probable it was, a mollusk, it must have been one of advanced type, and with a highly complex structure, as well as the singular apparatus for flotation implied in a chambered shell with a siphuncle.

Next to this among these primitive Mollusks are straight and spiral shells representing those delicate and beautiful animals of the modern seas, the Pteropods, or wing-footed Sea-snails, beautiful and graceful creatures, the butterflies of the sea, and moving in the water with the greatest ease and beauty by the aid of membranous fins, or wings, sometimes brightly coloured. These creatures abound in all latitudes in the modern ocean, and their delicate shells sometimes accumulate in beds of "Pteropod sand." They very early entered on the arena of marine life, and have continued to this day.

We miss here the two great Molluscan groups of the creeping Sea-snails like the limpet and whelk, and of the ordinary bivalves like the oyster and cockle. Both are present in the lowest Cambrian, though in small numbers compared with their present abundance. Possibly they had not yet appeared in the Etcheminian Sea, though the muddy and sandy bottoms, evidenced by its slates and sandstones, would seem to have afforded favourable habitats, and warrant the expectation that species may yet be found.

The case was different with the little group of the Lamp-shells, or Brachiopods. These creatures, somewhat resembling the ordinary bivalves in their shelly coverings, were very dissimilar in their internal structure, and once settled on the bottom they were attached for life, not having even the limited means of locomotion possessed by the Sea-snails and common bivalves. They collected their food wholly by means of currents of water produced by cilia, or movable threads, on arms or processes within their shells. In this they resembled the young or embryo stages of some of the more ordinary Mollusks, though they are so remote from these in their adult condition that they have usually been placed in a distinct class, and some naturalists have thought it best to separate them from the Mollusks altogether. Their history is peculiar. Coming into existence at a very early date, they became very abundant in early Palæozoic times, then gradually gave place to the ordinary bivalves, and in the modern seas are represented by very few species. Yet while in the middle period of their history they are represented by very many peculiar specific and generic forms. Some of the earliest types, like Obolus and Lingula, persist very long, and the latter has continued without change from the Early Cambrian to the Modern period.

The great group of the Sea-stars and Sea-urchins appears only in a few of its lower forms, and seems to be the only class represented by embryonic types. The coral animals are absent, so far as known. The Jelly-fishes and their allies cannot be preserved as fossils, but some peculiar markings, at one time regarded as plants, are now supposed to be trails made by the tentacles of creatures of this kind moving over muddy bottoms. A few spicules indicate Sponges, and the ubiquitous groups of the marine Protozoa, the Foraminifera and the Radiolaunus, are represented by shells scarcely distinguishable from those of modern species. The great and peculiar forms represented at this early time by Cryptozoon and its allies seem long ago to have perished, and we shall have to return to them in a later stage of our inquiry.

To sum up the little that we know of this earliest Palæozoic life:—It was perfect of its kind, equally pregnant with evidences of design, and of the nicest and most delicate contrivance as the animal life of any later time, and it presupposed vegetable life and multitudes of minute organic beings altogether unknown to us to nourish the creatures we do know. As an example of this, a little Brachiopod or sponge nourished by the currents produced by its cilia, or a Jelly-fish gathering food by its thread-like tentacles, or a Globigerina selecting its nourishment by its delicate gelatinous pseudopods, required an ocean swarming with minute forms of life, which probably can never be known to us, but every one of which must have been an inscrutable miracle of organization and vital function.

Lastly, with reference to our present subject, the Etcheminian fossils carry life backward one whole great period earlier than the Lower Cambrian, and appear to indicate that we are approaching a beginning of living things in the Palæozoic world. Much no doubt remains to be discovered, but it would seem that any future discoveries must fail to negative this conclusion.

The Huronian.

In whatever way the rocks immediately below the Cambrian may be classified, it is certain that the next system in descending order is that to which Logan long ago gave the name Huronian, from its development on Lake Huron[12]—a name to which it is still entitled, though there may, perhaps, be some grounds for dividing it into an upper and lower member.[13] To this sub-division, however, we need not for the present give any special attention. In the typical area of Lake Huron the Huronian consists of quartzites, which are merely hardened sandstones, of slates which are muddy or volcanic-ash beds, of conglomerates or pebble-rocks, and of coarse earthy limestone. With these rocks are deposits of igneous material which represent contemporary volcanic eruptions. In other districts, as in New Brunswick, Newfoundland, etc., the beds have been considerably altered, and are locally more mixed with igneous products. The physical picture presented to us by the Huronian is that of a shore deposit, formed under circumstances in which beds of pebbles and sand were intermixed with the products of neighbouring volcanoes.

[12] Dr. G. M. Dawson, F.R.S., the present Director of the Geological Survey of Canada, whose judgment in this matter should be of the highest value, holds that the original simple arrangement of Logan still holds, notwithstanding the multitude of new names proposed by the Western Geologists of the United States.

[13] Van Hise, "Pre-Cambrian Rocks of North America." Comptes Rendus, 5th Session International Geol. Congress 1891, p. 134. Also "Report U.S. Geol. Survey, 1895."

Fig. 15.—Annelid Burrows, Hastings Series, Madoc.
1. Transverse section of Worm-burrow—magnified, as a transparent object. (a) Calcareo-silicious rock. (b) Space filled with calcareous spar, (c) Sand agglutinated and stained black. (d) Sand less agglutinated and uncoloured. 2. Transverse section of Worm-burrow on weathered surface, natural size. 3. The same, magnified.

Such a formation is not likely to afford fossils in any considerable number and variety, even if deposited at a time of abundant marine life. It is therefore not wonderful that we find little evidence of living beings in the Huronian. In Canada I can point to nothing of this kind, except a few cylindrical burrows, probably of worms ([Fig. 15]), and spicules possibly of silicious sponges, which occur in nodules of chert in the limestones, traces of laminated forms like Cryptozoon or Eozoon ([Fig. 17]), and minute carbonaceous fragments which may be debris of sea-weeds or Zoophytes. In rocks of similar age in the United States, Gresley has recently discovered worm-burrows, and in Brittany there are quartzite beds in which Barrois and Cayeux believe that they have found tests of Radiolarians, Foraminifera and spicules of sponges, but their organic nature has been denied by Rauff, of Bonn. The casts of Foraminifera, however, at least appear to be organic ([Fig. 16]), and it is quite likely that Cayeux may be able to verify his Radiolarians and sponges as well. Matthew's observations in New Brunswick in any case establish their probability. Gümbel also recognises a species of Eozoon in the equivalent rocks of Bavaria (see [p. 213]).

Fig. 16.—Casts of Foraminifera, from the Huronian of Brittany. (After Cayeux.)
Compare with Globigerinæ on [Fig. 12] and Archæospherinæ, Figs. [50]-[54].

Fig. 17.—Cryptozoon or Eozoon from the Hastings Series, Tudor, Ontario (natural size).
From a specimen collected by the late Mr. Vennor, and now in the collection of the Geological Survey, Ottawa. (See also Frontispiece and figure of Eozoon Bavaricum, [p. 213].)

It is evident that here we have approached the limit of the higher forms of marine invertebrate life, having as yet nothing to show except worms and Protozoa. It is to be observed, however, that there may be somewhere Huronian deposits formed in deep and quiet waters, which may give better results, and that the unconformity between the Huronian and overlying Kewenian may indicate a lapse of time, of which monuments may yet be found.

The Laurentian.

Last of all we have the widely distributed Laurentian system of Logan, the oldest known to geologists, and which with the Huronian constitutes the great Archæan group of formations of Dana and others. In its lowest part this consists entirely of the stratified granitic rock known as gneiss, inter-bedded in some places with dark-coloured crystalline rocks or schists. This may be a part of the first-formed crust of our globe, produced under conditions different from those of any later rocks, and incompatible with the existence of life. The upper part of the Laurentian system, however, known in Canada as the "Grenville Series," shows evidence of ordinary marine deposition in quiet waters, which may have been not unfavourable to the lower forms of marine life; and though its beds have been greatly changed by heat and pressure, we can still to some extent realize the conditions of a time of comparative quiescence intervening between the underlying Lower Laurentian and the succeeding Huronian. This part of the system still contains gneisses, bedded diorites, and other rocks which may have been volcanic; but it has also quartzites and quartzose gneisses which must have been sandstones or shales, thick limestones, beds of carbon now in the state of graphite or plumbago, and large beds of iron ore. Such rocks were in all succeeding formations produced under water and by accumulations of the remains of plants and the hard parts of animals, in strictly sedimentary beds, usually formed slowly and without mechanical disturbance. Hence we may infer that aquatic life at least existed in this early period, and as there must have been land and water, shallows and deep seas, there may have been scope for various kinds of living beings. The Grenville period is, however, separated from the succeeding Huronian by a great interval, occupied mainly by volcanic ejections and earth-movements; so that our Grenville series, if it contains organic remains, may be supposed to afford species differing from those of the Huronian, and to form a sort of oasis in the desert of the early pre-Cambrian world. We find that the limestones of this age actually contain remains supposed to be of animal origin. They were first found in Canada, which contains the largest and best exposed area of these rocks in the world, and were brought under the notice of geologists by the late Sir William E. Logan, the first director of the Geological Survey of that country.

In anticipation of details to be given later, the story of this discovery and its announcement may here be given in brief

As early as 1858, Sir William Logan had begun to suspect that certain laminated bodies found in the Laurentian limestones of the Grenville series might be of organic origin. The points which struck him were these: They differed from any known laminated concretions; they resembled the "Stromatoporæ" or layer-corals of the lower Palæozoic rocks next in succession to the Laurentian and Huronian; the forms were similar in all the specimens, while the mineralizing substances were different; they were found only in the limestone, and specially in one of the three great beds known in the formation, the upper limestone of the Grenville system. He exhibited specimens, and mentioned these probabilities at the meeting of the American Association in 1859. In 1862 it was suggested to Logan that the microscopic structure of some of the best preserved examples should be studied, and slices were accordingly prepared and submitted to the writer for examination. They revealed in the calcareous laminæ of the specimens complicated systems of canals or tubes filled with mineral matter, which appeared to be similar to those that Carpenter had recognised in the thickened parts of the shells of modern Foraminifera. This clew being followed, large numbers of slices of the supposed fossils and of the containing limestone and of similar limestones from other parts of the world were examined.

The writer also visited the localities of "Eozoon," and studied its mode of occurrence in situ. The facts ascertained were communicated to the Geological Society of London, the name "Eozoon Canadense" being proposed for the species. Its description was accompanied by a paper on the geological conditions by Logan, and one on the chemical conditions by Sterry Hunt, while supplementary notes were added by the late Dr. Carpenter and Professor T. Rupert Jones. Thus launched on the scientific world, "Eozoon" at once became a fertile subject of discussion, and volumes of more or less controversial literature have appeared respecting it. It still has its friends and opponents, and this may long continue, as so few scientific men are sufficiently acquainted on the one hand with the possibilities and conditions of the preservation of fossils in crystalline rocks, and on the other hand with the structures of modern "Protozoa." Thus, few are in a position to form an independent judgment, and "Eozoon" has met with some scepticism on the part both of biological and mineralogical specialists.

To aid us in forming an opinion, it will be necessary to consider the oldest known strata of the earth's crust, and the evidence which they afford of the condition of the world when they were deposited. As preliminary to this, we may look at the following table of pre-Cambrian formations in Canada.

SUCCESSION OF PRE-CAMBRIAN ROCKS IN
CANADA, AS UNDERSTOOD UP TO 1896.

(In descending order.)

PALÆOZOIC.		Etcheminian in New Brunswick, Kewenian or Upper Copper-bearing Series of Lake Superior, Signal Hill Series of Newfoundland. Chuar, and Grand Cañon rocks of Colorado, etc. Red and greenish Sandstones and Shales, Conglomerates, Igneous Outflows and Ash-rocks. Bivalve Crustacea, Mollusks, Worms, Sponges, Cystideans, Zoophytes, Protozoa, Cryptozoon.
		(Unconformity.)
EOZOIC.		Huronian, including Hastings of Ontario, Coldbrook and Coastal of New Brunswick, Algonkian (in part). Conglomerates, Hard Sandstones, Shales and Schists, Iron Ores, Coarse Limestones, Igneous Outflows, and Ash-rocks. Worms, Sponges, Zoophytes, and Protozoa (Cryptozoon or Eozoon).
		(Unconformity [?])
EOZOIC.		Grenvillian or Upper Laurentian. Gneiss, Hornblendic and Micaceous Schists, Limestones, Quartzite, Iron Ores, Graphite. Eozoon, Archæozoon, Archæospherinæ, Archæophyton.
		(Unconformity.)
AZOIC		Archæan or Lower Laurentian. Gneiss, Hornblende Schists, with many igneous or igneo-aqueous intrusions.

THE FOUNDATIONS OF THE CONTINENTS, AND
THEIR GENERAL TESTIMONY AS TO LIFE