Part 5

(a) Lower acervuline portion. (b) Upper part, with three of the radiating laminæ and section of pores, (c) Portion of lamina, with pores, the calcareous skeleton unshaded.]

These are the humblest of all the inhabitants of the sea, presenting very simple, jelly-like bodies with few organs, but sometimes producing complex and beautiful calcareous and siliceous coverings or tests. Animals of this type have been found in the Lower Cambrian, though not in such vast multitudes as in some later formations. There are also in the Cambrian some large, laminated, calcareous bodies (Cryptozoon of Hall), to be noticed more fully below, and which have recently been traced in still lower deposits even below the lowest Cambrian (Figs. 7, 8). These have some resemblance to the layer-corals or stromatoporæ of the Silurian and Ordovician, which are by many regarded as the skeletons of coral animals of a low type; but the microscopic structure of Cryptozoon rather allies it with some of the larger forms of Protozoa found higher up in the series of formations. We shall have to discuss this later in connection with still older fossils.

Portion of slab reduced in size. (After Hall.) See also Fig. 59, p. 237.]

(a) Corneous layers, (a¹) One of these dividing, (b) Intermediate stroma with granules of calcite, dolomite and quartz, traversed by canals.

From a Micro-photograph by Prof. Penhallow.

If now in imagination we cast our tow-net or dredge into the sea of the Lower Cambrian, we may hope to take specimens illustrative of all our six groups of invertebrate animals, and under several of them examples of more than one subordinate group. Of the Crustaceans we might have representatives of four or five ordinal groups, and of the Mollusca as many. These are the two highest and most complicated. In the four lower groups we would naturally have less variety, though it would seem strange, were it not for so many examples in later periods, that the dominant and highest groups should be most developed in regard to the number of their modifications.

Specimen in Peter Redpath Museum.]

Of the whole we might perhaps have been able to secure at least 200 species even in one locality. The likelihood is that if there had been a collecting expedition like that of the Challenger in Early Cambrian times, it could have secured thousands of specific forms representing all the above types, more especially as we probably know very little of the softer and shell-less animals of these old seas, and there is some reason to believe that these may have been in greater proportion than in the present ocean.

In illustration of the richness of some parts of the lowest Cambrian sea, I may refer here to the large and beautifully illustrated Memoir of Walcott on the Lower Cambrian, containing fifty folio plates of species collected in a few districts of North America; and, as a minor example, to the contents of a loose boulder of limestone of that age, found at Little Metis on the Lower St. Lawrence, under the following circumstances (Fig. 9):--

Along what is now the valley of the Lower St. Lawrence and the gulf of the same name, there seem to have been deposited in the oldest Cambrian or Olenellus period beds of limestone rich in shells of marine animals and fragments of these. These can be seen in place in some parts of Newfoundland, and here and there on the hills bounding the St Lawrence River; but for the most part they have been swept away by the sea when these districts were being elevated to form parts of the American land. Their ruins appear as boulders and pebbles in thick beds of conglomerate or pudding-stone, constituting portions of the Upper Cambrian and Lower Ordovician series, which now occupy the south coast of the Lower St. Lawrence. In one of these boulders, less than a foot in diameter, removed from its hard matrix and carefully broken up, I found fragments representing eleven different species, of which no less than eight were trilobites, one a gastropod, one a brachiopod, and one probably a sponge--and this forms an interesting illustration of the number of species sometimes to be found in a limited space, and also of the great prevalence of the Trilobites in these beds. The statistics of these groups for North America, as given by Walcott, show 165 species belonging to all the groups enumerated above, and of these the Trilobita constitute one-third of the whole; so that the Olenellus Zone, as it has been called from one genus of these Crustaceans, might well be named the reign of Trilobites, unless, indeed, as the indications already referred to seem to show, giant cuttle-fishes, destitute of shells, were then the tyrants of the sea, but are represented only by the markings of their long and muscular arms on the soft sea mud while dashing after their Crustacean prey. What I desire, however, chiefly to emphasize is, that in the lowest beds of the Cambrian we have evidence of sea-bottoms swarming with representatives of all the leading types of marine invertebrate life, and therefore seem to be still far from the beginning of living things, if that was a slow and gradual process, rather than a sudden or rapid series of events.

PRE-CAMBRIAN LIFE

III

PRE-CAMBRIAN LIFE

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 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.

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.

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. 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).

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).

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.

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, 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).

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; and the black shales of the head of Lake Superior contain some impressions supposed to be trails of animals.