Philip Gosse - Omphalos - An Attempt to Untie the Geological Knot

On the western escarpment of the Pennine ridge, just as the traveller is entering Westmoreland, he would detect the bottom of the limestone; and here he would have an opportunity of seeing, what is rare in these parts, a stratum of the old red sandstone, lying between the former and the slaty rocks of the Cumbrian formations. And here at length, in the wild and magnificent scenery of these mountains, he sees the primitive and transition series, the greenstone, the sienite, and the granite, each of which is discernible in succession on the face of one or other of the lofty Fells of Cumberland.

Our traveller now comes home, and, musing on what he has seen, counts up some thirty or more distinct strata lying in regular succession one on another. But he has not seen all the world, nor even all England; but he reads the results of many independent observations, and finds that while, for the most part, the strata which he has seen are common to the whole surface of the globe, and while the order of their superposition is invariable everywhere, others are in some parts added, while perhaps some of those which he has observed are locally absent. Thus he is able to form a more distinct idea of the stratification of the earth's crust as a whole. It is composed of about forty distinct formations, generally increasing in thickness as we go downwards, so that the whole cannot be much less than ten miles in depth, supposing them in any locality to be all present, and to be lying in the horizontal plane.

Mathematicians have satisfactorily determined that the mean density of the globe is about five-and-a-half times that of water, or about twice that of granite, a fact inconsistent with any other supposition than that the interior is occupied by substances maintained in a fluid state by intense heat. The lowest point that has yet been patent to human observation is occupied by the granite, a compound rock, which bears evident marks of having been once in a state of fusion, and of having cooled slowly, and that under immense pressure, contracting and crystallizing as it parted with its heat. There is every reason to believe that the granite is not defined at its inferior surface, but that it merges into the molten mass, probably still solidifying.

After the outer portion of the granite had cooled sufficiently to become solid, there is evidence that it was covered by water, agitated by powerful currents, and probably in a heated state. The action of these currents disintegrated the rock, and deposited the constituent substances at the bottom of the sea – on the surface, and in the hollows, of the granite. For there is reason to think that the contraction of the primitive rock in the process of cooling, produced irregular undulations or crumplings of the surface, and frequent fractures and dislocations, elevating some parts and depressing others. The gneiss, the mica-schist, and the clay-slate, which are found immediately overlying the granitic rock in strata of vast thickness, are but the components of granite, separated and rearranged. "If we imagine common granite coarsely pounded, and thrown into a vessel of water, it will arrange itself at the bottom of the vessel in a condition very much like that of gneiss, which is indeed nothing else than stratified granite. If the water in which the pounded rock is thrown is moving along at a slow rate, and the clayey portion of the granite, called felspar , happens to be somewhat decomposed, as it often is, then the felspar (which is so truly clay that it makes the best possible material for the use of the potteries) and the thin shining plates of mica, will be carried further by the water than the lumps of white quartz or flint sand, which, with the other two ingredients, made up the granite; and the two former will be deposited in layers, which, by passing a galvanic current through them, would in time become mica-schist. If the mica were absent, or if the clay were deposited without it, owing to any cause, then a similar galvanic current would turn the deposit into something like clay-slate." 25 25 Ansted's Ancient World, 18.

The deposition of these strata, being formed out of granite, supposes the pre-existence of that rock; and as they occur in vast thicknesses, even of many thousand feet, then separation, deposition, and reconsolidation must have occupied, however rapidly we may suppose the processes to have been accomplished, considerable periods of time.

In these lower rocks, no trace of organic remains has been found. The shoreless ocean that covered the cooling surface of the earth's crust, harboured no polype or sponge, no rhizopod or infusorium, and the angles and clefts of the granite were fringed by no fucus, or conferva: all was waste and void. And if certain parts were elevated above the waters, the bleak and barren points were not clothed with grass, or moss, or even a lichen, and no animal wandered over their ridges. Or, if such did exist, either in land or water, all vestiges of their presence have been destroyed by the agency of the intense heat that subsequently prevailed.

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 26 Ansted's Ancient World, 30.

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 Oniscidæ (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.