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).