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Edinburgh Past and Present


OUTLINE OF ITS GEOLOGY. 147 leaf upon leaf, and mark how they retain even yet the ripple-mark impressed upon them by the moving water when they were still soft sand and mud, Many a face of the rock is covered with the trails of sea-worms which have left no other traces of their former existence. Were we to judge merely from the scarcity of fossils in these rocks, we might infer that the waters of the sea were not very prolific of life. Yet some of the beds of black and coal-like shale are crowded with- remains of gmptuZittes-slim grass-like stalks, each with a single or double row of close-fitting cells, in which separate individuals of a simple form of animal life now extinct once lived. These gruptuZifes, of which many species have been described, are almost the only fossils found among the Lammermuir and Moorfoot Hills. They are characteristic of that period of geological time to which the name of Silurian has been given. Before the close of this period, when a depth of many thousand feet of sand, mud, and gravel had been accumulated over the sea-bottom, ode of those great changes took place by which the crust of the earth has from time to time been affected. The vast mass of submarine sediment was squeezed and crumpled in such a way that the beds, originally horizontal, came to stand on end, and to be folded over and over like so many piles of carpets. It was this subterranean movement, prolonged probably through a succession of geological ages, which upbeaved the mass of land that has been carved into the present Highlands and the uplands of the Southern counties. But though some parts of the sea-floor were no doubt soon raised into land, and though as the subterranean movements continued the extent of land probably grew in proportion, the same ocean, with many of the same inhabitants, still lay beyond. Here and there, too, it ran in bays and channels into the new land. Among the Pentland Hills, for example, in the now hardened and broken sediments of' its bottom, occur the remains of small sponges, corals, crinoids, trilobites, brachiopods, lamellibranchs, and cephalopods. These fossils are crowded thickly together in certain bands of rock, while in others they occur but rarely. They agree generally with those found in the Ludlow and Wenlock formations of the upper Silurian series of England and Wales. The underground movements seem to have continued not only to the close of the Silurian period, but far into the next great chapter of geological t i m e %hat of the Lower Old Red Sandstone. The sea-bottom over the area of Britain was thereby raised into an irregular mass of land with wide inland seas or lakes, some of which may still have retained a communication with the open ocean. In those enclosed sheets of water the characteristic con- Its waters in some places teemed with life.
Volume 11 Page 206
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Volume 11 Page 207
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