A like phenomenon is exhibited in every mountainous country, as, for example, in the European Alps; but we need not go farther than the north of England for its illustration. Thus in Lancashire and central England the thickness of the Carboniferous formation, including the Millstone Grit and Yoredale beds, is computed to be more than 18,000 feet; to this we may add the Mountain Limestone, at least 2000 feet in thickness, and the overlying Permian and Triassic formations, 3000 or 4000 feet thick. How then does it happen that the loftiest hills of Yorkshire and Lancashire, instead of being 24,000 feet high, never rise above 3000 feet? For here, as before pointed out in the Alleghenies, all the great thicknesses are sometimes found in close approximation and in a region only a few miles in diameter. It is true that these same sets of strata do not preserve their full force when followed for indefinite distances. Thus the 18,000 feet of Carboniferous grits and shales in Lancashire, before alluded to, gradually thin out, as Mr. Hull has shown, as they extend southward, by attenuation or original deficiency of sediment, and not in consequence of subsequent denudation, so that when we have followed them for about 100 miles into Leicestershire, they have dwindled away to a thickness of only 3000 feet. In the same region the Carboniferous limestone attains so unusual a thickness—namely, more than 4000 feet—as to appear to compensate in some measure for the deficiency of contemporaneous sedimentary rock.[1] It is admitted that when two formations are unconformable their fossil remains almost always differ considerably. The break in the continuity of the organic forms seems connected with a great lapse of time, and the same interval has allowed extensive disturbance of the strata, and removal of parts of them by denudation, to take place. The more we extend our investigations the more numerous do the proofs of these breaks become, and they extend to the most ancient rocks yet discovered. The oldest examples yet brought to light in the British Isles are on the borders of Rosshire and Sutherlandshire, and have been well described by Sir Roderick Murchison, by whom their chronological relations were admirably worked out, and proved to be very different from those which previous observers had imagined them to be. I had an opportunity in the autumn of 1869 of verifying the splendid section given in Fig. 82 by climbing in a few hours from the banks of Loch Assynt to the summit of the mountain called Queenaig, 2673 feet high. The formations 1, 2, 3, the Laurentian, Cambrian, and Silurian, to be explained in Chapters XXV and XXVI, not only occur in succession in this one mountain, but their unconformable junctions are distinctly exposed to view. Fig. 82: Unconformable Palæozoic stata, Sutherlandshire (Murchison).