THE PRIMORDIAL, OR CAMBRIAN AGE.
Between the time when Eozoon Canadense flourished in the seas of the Laurentian period, and the age which we have been in the habit of calling Primordial, or Cambrian, a great gap evidently exists in our knowledge of the succession of life on both of the continents, representing a vast lapse of time, in which the beds of the Upper Laurentian were deposited, and in which the Laurentian sediments were altered, contorted, and upheaved, before another immense series of beds, the Huronian, or Lower Cambrian, was formed in the bottom of the sea. Eozoon and its companions occur in the Lower Laurentian. The Upper Laurentian has afforded no evidence of life; and even those conditions from which we could infer life are absent. The Lowest Cambrian, as we shall see, presents only a few traces of living beings. Still, the physical history of this interval must have been most important. The wide level bottom of the Laurentian sea was broken up and thrown into those bold ridges which were to constitute the nuclei of the existing continents. Along the borders of these new-made lands intense volcanic eruptions broke forth, producing great quantities of lava and scoriæ and huge beds of conglomerate and volcanic ash, which are characteristic features of the older Cambrian in both hemispheres. Such conditions, undoubtedly not favourable to life, seem to have prevailed, and extended their influence very widely, so that the sediments of this period are among the most barren in fossils of any in the crust of the earth. If any quiet undisturbed spots existed in which the Lower Laurentian life could be continued and extended in preparation for the next period, we have yet discovered few of them. The experience of other geological periods would, however, entitle us to look for such oases in the Lower Cambrian desert, and to expect to find there some connecting links between the life of the Eozoic and the very dissimilar fauna of the Primordial.
The western hemisphere, where the Laurentian is so well represented, is especially unproductive in fossils of the immediately succeeding period. The only known exception is the occurrence of Eozoon and of apparent casts of worm-burrows in rocks at Madoc in Canada, overlying the Laurentian, and believed to be of Huronian age, and certain obscure fossils of uncertain affinities, recently detected by Mr. Billings, in rocks supposed to be of this age, in Newfoundland. Here, however, the European series comes in to give us some small help. Gümbel has described in Bavaria a great series of gneissic rocks corresponding to the Laurentian, or at least to the lower part of it; above these are what he calls the Hercynian mica-slate and primitive clay-slate, in the latter of which he finds a peculiar species of Eozoon, which he names Eozoon Bavaricum. In England also the Longmynd groups of rocks in Shropshire and in Wales appears to be the immediate successor to the Upper Laurentian; and it has afforded some obscure “worm-burrows” or, perhaps, casts of sponges or fucoids, with a small shell of the genus Lingulella, and also fragments of crustaceans (Palæeopyge). The “Fucoid Sandstones” of Sweden, believed to be of similar age, afford traces of marine plants and burrows of worms, while the Harlech beds of Wales have afforded to Mr. Hicks a considerable number of fossil animals, not very dissimilar from those of the Upper Cambrian. If these rocks are really the next in order to the Eozoic, they show a marked advance in life immediately on the commencement of the Primordial period. In Ireland, the curious Oldhamia, noticed below, appears to occur in rocks equally old. As we ascend, however, into the Middle and Upper parts of the Cambrian, the Menevian and Lingula flag-beds of Britain, and their equivalents in Bohemia and Scandinavia, and the Acadian and Potsdam groups of America, we find a rich and increasing abundance of animal remains, constituting the first Primordial fauna of Barrande.
The rocks of the Primordial are principally sandy and argillaceous, forming flags and slates, without thick limestones, and often through great thicknesses, very destitute of organic remains, but presenting some layers, especially in their upward extension, crowded with fossils. These are no longer mere Protozoa, but include representatives of all the great groups of animals which yet exist, except the vertebrates. We shall not attempt any systematic classification of these; but, casting our dredge and tow-net into the Primordial sea, examine what we collect, rather in the order of relative abundance than of classification.
Over great breadths of the sea bottom we find vast numbers of little bivalve shells of the form and size of a finger-nail, fastened by fleshy peduncles imbedded in the sand or mud; and thus anchored, collecting their food by a pair of fringed arms from the minute animals and plants which swarm in the surrounding waters. These are the Lingulæ, from the abundance of which some of the Primordial beds have received in England and Wales the name of Lingula flags. In America, in like manner, in some beds near St. John, New Brunswick, the valves of these shells are so abundant as to constitute at least half of the material of the bed; and alike in Europe and America, Lingula and allied forms are among the most abundant Primordial fossils. The Lingulæ are usually reckoned to belong to the great sub-kingdom of mollusks, which includes all the bivalve and univalve shell-fish, and several other groups of creatures; but an able American naturalist, Mr. Morse, has recently shown that they have many points of resemblance to the worms; and thus, perhaps, constitute one of those curious old-fashioned “comprehensive” types, as they have been called, which present resemblances to groups of creatures, in more modern times quite distinct from each other. He has also found that the modern Lingulæ are very tenacious of life, and capable of suiting themselves to different circumstances, a fact which, perhaps, has some connection with their long persistence in geological time. They are in any case members of the group of lamp-shells, creatures specially numerous and important in the earlier geological ages.
Fig. 8.—LIFE IN THE PRIMORDIAL SEA.
On the bottom are seen, proceeding from left to right, Oldhamia antiqua, Lingulæ, Arenicolæ, Oldhamia radiata, Paradoxides, Histioderma, Agnostus, Oldhamia radiata, Algæ, and Lingulæ. In the water are Hymenocaris, different species of Trilobites, and Pteropods.