Having thus traced the interesting series of geographical conditions indicated by the Silurian series, we may next take our station on one of the submerged plateaus, and inquire as to the new forms of life now introduced to our notice; and in doing so shall include the life of both the Lower and Upper Silurian.
Fig. 9.—Fragment of Lower Silurian Limestone, sliced and magnified ten diameters, showing the manner in which it is made up of fragments of corals, crinoids, and shells. (From a paper oil the Microscopic Structure of Canadian limestone, “Canadian Naturalist.”)
First, we may remark the vast abundance and variety of corals. The polyps, close relatives of the common sea-anemone of our coasts, which build up our modern coral reefs, were represented in the Silurian seas by a great number of allied yet different forms, equally effectual in the great work of secreting carbonate of lime in stony masses, and therefore in the building-up of continents. Let us note some of the differences. In the first place, whereas our modern coral-workers can show us but the topmost pinnacles of their creations, peeping above the surface of the sea in coral reefs and islands, the work of the coral animals of the Silurian has been finished, by these limestones being covered with masses of new sediment consolidated into hard rock, and raised out of the sea to constitute a parfc of the dry land. In the Silurian limestones we thus have, not merely the coral reefs, but the wide beds of comminuted coral, mixed with the remains of other animals, which are necessarily accumulated in the ocean bed around the reefs and islands. Further, these beds, which we might find loose and unconsolidated in the modern sea, have their fragments closely cemented together in the old limestones. The nature of this difference can be well seen by comparing a fragment of modern coral or shell limestone from Bermuda, with a similar fragment of the Trenton limestone, both being sliced for examination under the microscope. The old limestone is black or greyish, the modern one is nearly white, because in the former the organic matter in the animal fragments has been carbonised or converted into coaly and bituminous matter. The old limestone is much more dense and compact, partly because its materials have been more closely compressed by superincumbent weight, but chiefly because calcareous matter in solution in water has penetrated all the interstices, and filled them up with a deposit of crystalline limestone. In examining a slice, however, under the microscope, it will be seen that the fragments of corals and other organisms are as distinct and well preserved as in the crumbling modern rock, except that they are perfectly imbedded in a paste of clear transparent limestone, or rather calcareous spar, infiltrated between them. I have examined great numbers of slices of these limestones, ever with new wonder at the packing of the organic fragments which they present. The hard marble-like limestones used for building in the Silurian districts of Europe and America, are thus in most cases consolidated masses of organic fragments.
In the next place, the animals themselves must have differed somewhat from their modern successors. This we gather from the structure of their stony cells, which present points of difference indicating corresponding difference of detail in the soft parts. Zoologists thus separate the rugose or wrinkled corals and the tabulate or floored corals of the Silurian from those of the modern seas. The former must have been more like the ordinary coral animals; the latter were very peculiar, more especially in the close union of the cells, and in the transverse floors which they were in the habit of building across these cells as they grew in height. They presented, however, all the forms of our modern corals. Some were rounded and massive in form, others delicate and branching. Some were solitary or detached, others aggregative in communities. Some had the individual animals large and probably showy, others had them of microscopic size. Perhaps the most remarkable of all is the American Beatricea,[H] which grew like a great trunk of a tree twenty feet or more in height, its solitary animal at the top like a pillar-saint, though no doubt more appropriate and comfortable; and multitudes of delicate and encrusting corals clinging like mosses or lichens to its sides. This creature belongs to the very middle of the Silurian, and must have lived in great depths, undisturbed by swell or breakers, and sheltering vast multitudes of other creatures in its stony colonnades.
[H] First described by Mr. Billings. It has been regarded as a plant, and as a cephalopod shell; but I believe it was a coral allied to Cystiphyllum.
Fig. 10.—LIFE IN THE SILURIAN AGE.