Page:Popular Science Monthly Volume 8.djvu/180

168 resemblance to a coral-reef, instead of the aggregate of minute and separate shells which formed the old Chalk, and which is even now continuing the like formation. I do not know and more remarkable result of microscopic inquiry, than the very distinct evidence it has afforded, in well-preserved specimens of this Eozoön Canadense, of a minutely tubular structure, which my own researches into the structure of the Foraminifera enable me to identify with certainty as belonging to that type. For we are thus carried back in geological time to a period so extremely remote, that (as Sir William Logan remarked) the oldest fossils previously known are modern in comparison. The investigations of Sir Roderick Murchison have shown that the equivalent of the Laurentian in this country is the "fundamental gneiss" of Scotland, which (as I was shown a few days ago by my friend Mr. Symonds, of Pendock) crops up in the Malvern Hills. Now, in Central Europe this fundamental gneiss has a thickness of 90,000 feet; and near its base Prof. Gümbel has recognized the equivalent of the Canadian Eozoön, which must have thus preceded the life of what has been called the "primordial zone," corresponding to our Cambrian rocks, by an interval of time so great that no geologist would venture to assign a limit to it.

The Cambrian series, consisting of the grits, sandstones, and slates, that form the mountains of North Wales, scarcely contain any limestone; and we may pass from this to the Silurian, or Mid-Wales, series in which we have the well-known Dudley limestone, as well as other less important seams. A slab of Dudley limestone usually shows an extraordinary variety of fossils, among which the most conspicuous are generally the beaded stems of Encrinites; the joints of these stems, when separated by the weathering of the rock, being known in the north as "St. Cuthbert's beads." The whole of this limestone is obviously made up of the corals, shells, crinoids, etc., which we, find imbedded in it, and of a matrix formed by comminuted fragments of the like types. A much greater development of these calcareous beds presents itself in North America, the Trenton limestone occurring in the lower Silurians, and the Niagara limestone in the upper; and these rocks have obviously been formed by the same agency as the Dudley limestone.

Passing on now to the Devonian series, we find beds of limestone interposed among the sandstones, shales, and' conglomerates, of which it is chiefly composed; and these, like the Silurian limestones, are made up of the fossilized remains of corals, shells, crinoids, etc., more or less resembling those of earlier age. It is on the Old Red Sandstone, which is here the uppermost member of the Devonian formation, that, as I have already pointed out, our Carboniferous series immediately rests; its lower beds being distinguished as "limestone shales," on account of the interposition of seams of shale (formed of a mixture of sand and clay) between the layers of limestone.