c. Yoredale Rocks, a series of flagstones, gritstones, limestones and shales, with seams of coal, occurring in the northern counties. It is underlain by—
d. Carboniferous or Mountain Limestone, which in places is upwards of 1,000 feet thick, and full of fossils. The stems of encrinites, or "stone-lilies," corals, brachiopods (e.g. Productus, Orthis, etc.), and Mollusca, including some Cephalopods, like Goniatites and the straight Nautilus (Orthoceras), with fish teeth, etc., go to compose this tough, bluish-grey limestone which is largely quarried for marble mantlepieces, etc.
e. The Tuedian group in the north, and Lower Limestone Shale in the south, follow next, and consist of shales, sandstones, limestones, and conglomerates, varying greatly in different districts, and containing few fossils.
3. Devonian or Old Red Sandstone. To this age are assigned a perplexing series of strata, the principal members of which consist of (a) a thick limestone, well seen in the cliffs and marble quarries of south Devon, and full of fossil-corals (e.g. Favosites polymorpha [or cervicornis]) Brachiopods, and Mollusca, etc.
b. A series of sandstones, slates, and limestones in North Devon containing Trilobites (Phacops, Bronteus, etc.), Brachiopods, and other fossils.
c. The Old Red Sandstone of Wales, the North of England, and Scotland, consisting of red and grey sandstone and marly beds, with remains of fish.
These fish, unlike most now living, were more or less covered with hard external plates, and possessed merely a cartilaginous skeleton. In one set of individuals, indeed (Pterichthys), the armour plates formed quite a little box. These creatures propelled themselves by means of two arm-like flippers, rather than fins. They were but a few inches long, and appear pigmies in contrast to the strange half-lobster-like crustacean Pterygotus, that lived with them, and attained sometimes as much as five feet in length.
4. Silurian. Named by Sir Roderick Murchison after a tribe of Ancient Britons that dwelt in that part of Wales, where these rocks were first observed. Some of Murchison's Lower Silurian beds were included by Professor Sedgwick in his Cambrian, of which we shall have to speak next; and as these two geologists never could agree on a divisional line between their respective formations, and since succeeding observers have followed sometimes one and sometimes the other method of classification, considerable confusion has resulted. Here, however, for several reasons, we propose to follow Sedgwick's arrangement; and hence, under the term Silurian, retain only Murchison's Upper beds. They consist of a series of sandstones, gritstones, conglomerates, shales, limestones, etc.
Amongst the more important fossils, which are very abundant in the limestones, are various corals (e.g. the Chain-coral Halysites), Star-fish, Crinoids, Trilobites (Phacops, etc.), Polyzoa, Brachiopods and Mollusca, especially Cephalopoda (Orthoceras, Nautilus, etc.).
These rocks occur principally in the border land between England and Wales, and the adjacent counties; but are also represented in Westmoreland, Scotland, and Ireland. Their principal subdivisions are given in the Table on p. 16.