Fig. 655.—Eye of Trilobite.
Thus we see that ages of comparative quiet succeeded the first great contraction of the earth’s crust, probably millions of years, during which time the tides and currents of the ocean had to wash and wear down all the thousands of projecting rocks or inequalities, and dissolve (as before described) all the lime, depositing the sand and clay in those immense strata which form the old-named “transition series”; this appears to have taken place over nearly the whole world at that time, and ages upon ages must have elapsed to form such deposits as the sandstone, claystone, and limestone, in alternation, forming the “Llandilo,” “Caradoc,” and “Wenlock” strata, more than a mile in thickness; these are by some geologists reckoned among the primary series (by some called the “transition rocks”), and in England form the “Cambrian” and “Silurian” systems which are so rich in minerals and metallic veins.
Fig. 656.—Limestone made up of encrinite.
The Old Red Sandstone, or Devonian System, is the next of the series of layers which built up our earth, but a great gap of years separates it from the Silurian. We find the “Old Red” in the Mendips and in Scotland. The rocks in the West of England are apparently of later deposition and of marine origin, while the “Old Red” is apparently a fresh-water deposit. It is very “arenaceous,” and owes its tint chiefly to iron, although there are circumstances in which it appears neither as a sandstone nor with a red colour; but red sandstone describes the true formation very accurately.
Fig 657.—Graptolites.
In this water-deposited system, whether in lakes or by the sea, we find a considerable advance upon the Silurian. We have flora in more variety—seaweeds and ferns. The remains are all aquatic. We have nothing higher in the scale of creation than the fish, the first vertebrates; and judging by varieties a very considerable time must have elapsed during which the Old Red Sandstone was deposited. The dipterus—or double-winged—is herewith shown as an example of the fossil fish of the Old Red Sandstone period.
Fig. 658.—“Double-winged” Fish.