But, in the numerous strata that overlie the rocks of granitic origin, there are found, in varying abundance, proofs that, when they were deposited, the surface of our earth had become the abode of organic life. Zoophytes lived in the ocean, some of which were engaged in secreting lime from the water, and depositing it in coral-reefs; stalked and jointed Star-fishes waved like lilies of stone from the submerged rocks; Sea-worms twined over the mud; mailed Crustaceans swam to and fro; and Mollusks, both bivalve and univalve, crawled over the ledges or reposed in the crevices. The remains of these occur in the Silurian rocks that lie immediately on the primitive granitic formations of Cumberland and North Wales. The construction of the coral-reefs of that deposit, in particular, must have occupied a lengthened period, continuing to go on, "month after month, year after year, century after century, until at length the depth changed, in which they could most conveniently live, or, owing to some other cause, their labours were brought to a close, and they disappeared from amongst existing species."[26] [Illustration: A TRILOBITE. (_Calymene Blumenbachii._) _a._ extended; back view. _b._ rolled up; side view. _c._ rolled up; front view.] Not a single species, or even a single genus of those early strata, is identical with any that exists now. The Coral-polypes, for instance, while allied to ours, are quite distinct from them, though endowed with similar powers and habits, so that we may reason from analogy on the laws of their deposits. The Trilobites were allied to the tiny water-fleas (_Entomostraca_) of the present day: like the _Oniscidae_ (wood-lice, buttons, &c.) of our gardens, they had the habit of rolling their plated bodies into a ball. These are found in great numbers, their remains often heaped on one another. The Mollusca of those seas were chiefly of the class _Cephalopoda_--one of the least populous now-a-days, but then existing in vast number and variety; the Brachiopoda, Conchifera, and Gastropoda, were, however, well represented also. Such were the inhabitants of the sea during the Silurian period, in which a series of solid deposits were made, the aggregate, probably, exceeding 50,000 feet in thickness. Each deposit, though not more than a few inches in depth, "is provided with its own written story, its sacred memoranda, assuring us of the regularity and order that prevailed, and of the perfect uniformity of plan." Over all these, however, we see laid the strata of the Devonian system, especially the old red sandstone, which in some places attains a thickness of 10,000 feet. It is composed of a coarse agglomeration of broken fragments of the old granitic rocks, rolled and tossed about, apparently by the ever-breaking waves of shingle-beaches, until the hardest stones are worn into rounded pebbles by long and constant attrition.