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Edinburgh Past and Present


OUTLINE OF ITS GEOLOGY. 151 flanks the eastern side of the Pentland Hills, they have been thrown nearly od edge from Portobello southward by Edmonston, Gilmerton, Loanhead, Penicuik, and Brunston. Some thin bands of limestone with dwarf marine fossils overlie these coals. One of them is welt exposed at the Joppa quarry. Overlying this limestone series there is found a great mass of coarse sandstone and fine conglomerate, often red in colour, and known locally as the Roslin sandstone or Moor-rock. It attains a depth of 340 feet, and is believed to be the equivalent of the millstone grit of the English series. It forms the sides of the romantic ravine of the Fisk between Roslin and Lasswade. Next in order comes the highest section of the Carboniferous system, known as the Coal-Measures. It consists of sandstones, shales, fireclays, and coal-seams, and in the Mid-Lothian basin attains a thickness of 1590 feet. It seems to have been formed under circumstances not unlike those in which the Edgecoal series was laid down, with the exception that the marine limestones were not formed. The swamps were from time to time densely covered with vegetation, which, though generally agreeing with that of the older series, differs considerably in many of its species. These thick matted accumulations of vegetation form now the seams of coal, while the sandy and clayey strata between them represent the sediment laid down upon the submerged forests as each of these was successively carried down beneath the water. At this part of the history we come upon the greatest hiatus in the geological records of the district. A vast series of ages passed away, during which the physical geography of the area of Britain went through many vicissitudes, and the plants and animals alike of land and sea were completely changed. Yet of these events no geological memorial has been preserved at Edinburgh. We know from evidence elsewhere existing that long after our coal-fields were formed some of them were pierced by volcanoes. Those of Ayrshire, Lanarkshire, and Fife suffered in this way. Probably the upper part of Arthur's Seat belongs to that period of volcanic activity. At a far later, though still remote, time, a renewed outburst of the subterranean forces gave rise to the vast basaltic plateaux of Antrim, Mull, Eigg, Skye, and the Faroe IsLnds. When these western vents were still busy pouring out their masses of lava, the country, by some process as yet little understood, came to be cracked across in innumerable places, the fissures having on the whole an east and west trend, and increasing in number toward the volcanic district of Antrim and the Hebrides. Into these fissures the basalt f?om below rose, filling them and forming the long wall-like masses known as diies. Several of these dikes occur at or near Edinburgh.' Some are now concealed by the streets of the city. One
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152 EDINBURGH PAST AND PRESENT. traverses the coal-field near Niddry, while two very conspicuous examples run across the Carboniferous Limestone series near Prestongrange and Longniddry. Another great gap, for the filling up of whichno evidence exists in this part of the country, separates these volcanic rocks from the next geological events in our chronicle-those of the Ice Age. The neighbourhood of Edinburgh will always bear a special interest in regard to this part of geology, from the fact 'that it was here Sir James Hall observed and described, those dressed ' rock-surfaces which are now everywhere acknowledged to be due to the grindiog action of ice. They are to be seen on the west slope of Corstorphine Hill, on the top of the southern part of Salisbury Crags, on the sandstone at Joppa salt-pans, on the porphyrite at Blackford Quarry, on the top of Allermuir Hill, one of the Pentlands, at a height of 1617 feet, and in many other places. The general direction of the strk and groovings is a little to the north of east, indicating that the mass of ice which produced these markings moved seawards along the line of the valley of the Firth of Forth.. The roqks of the neighbourhood of Edinburgh pass beneath masses of glacial drift-the productof the glaciers, icebergs, and seas of the glacial period. At the bottom of these deposits lies the boulderclay or till-a stiff dark-blue clay stuck full of stones of all sizes, up to blocks of a yard in diameter. Many of these are well smoothed, and striated like the surfaces of the solid rocks underneath. On examination it is found that the majority of them are of local derivation, that some have come from distances of ten or fifteen miles to the west, a smaller proportion from western hills twenty or thirty miles away, while a very small number have travelled from the Highland mountains. Thus the stones corroborate the testimony of the striz on the rocks, that the general icemovement was here from the west. The gradual deflection to east-by-north ,was evidently due to the influence of the shape and direction of the great valley upon the march of the ice. While the lower parts of the boulderclay appear to have been formed under a huge sheet of land-ice moving steadily across the country, like the enormous icemantle of Greenland, the upper parts of the deposit suggest that they may have originated to some extent in the sea, either underdw solid ice or under the broken bergs which flanked the long front of the ice-sheet They contain larger blocks than the lower parts of the till, and these are often arranged in rude lines. The boulderclay, as might be expected, is singularly destitute of fossils. To the west of Edinburgh,; when the Union Canal was being cut, a well
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