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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.
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t 48 EDINBURGH PAST AND PRESENT. ~~ glomerates and sandstones of the Old Red Sandstone system were elaborated. They seem to have covered a large part of Western Europe, and to have extended. eastwards over much of the north of Russia. At the beginning’of the Old Red Sandstone perid, what is now the northern. half of Britain may have bekn connected with some large continental mass of land, but was certainly covered by wide sheets of water, out of which the high grounds rose as scattered groups of islands. This land too was slowly sinking down, so that the waters encroached more and more’ upon its shores, from which enormous quantities of gravel and sand were swept away to form the vast piles of conglomerate and sandstone. Some idea may be formed of the extent of this depression, and of the amount of detritus which must have been worn off the land, from the fact that the Old Red Sandstone, even now, after the lapse of so many ages of subsequent decay, is still often 15,000 feet thick, and in the east of Strathmore exceeds even 20,ooo feet. As a rule,’ the waters in which those deposits were laid down seem to have been rather unfavourable to life, at least organic remains are for the most part scarce, though here and there fishes occur abundantly. In the immediate neighbourhood of Edinburgh no fossils have yet been met with in the Old Red Sandstone except some traces of plants towards the top. In this district, however, considerable interest belongs to that system, owing to the large masses of volcanic rocks which are imbedded in it. The conglomerates and sandstones constitute the southern end of the Pentland Hills and stretch thhnce into the counties of Peebles and Idnark. But the main mass of these hills, including also the Braid Hills near Edinburgh, consists of various lavas and tuffs ekpted from volcanic vents which continued to be active during a long part of the Lower Old Red Sandstone period, The curious tuKlike masses of .the Braid Hills, as well as the thickening of the bvas towards the north, seem to point to one or more chief craters of emission having existed somewhere about the site of the Braid Hills. Owing to the deposit of later formations upon them; we cannot tell bow thick these piles of volcanic material may originally have been. But even now, the part which is seen would, if placed in its original position, make a mountain-ridge considerably higher than Beb Nevis, During the time when these Pentland volcanoes were at work there was a prodigious amount of volcanic activity over Scotland. From the ridge of the Highlands pormous streams of lava and showers of dust and stones were thrown into the waters which then spread over the centre of the country. From the high land of the southern uplands there would seem 10 hatre been a similar outpouring. Many thousands of feet of volcanic rock
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