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Edinburgh Past and Present


OUTLINE OF ITS GEOLOGY. '49 were thus accumulated, which though much obscured and broken by later geological changes, still form some of the most conspicuous hillsin the midland 'valley,-the. Sidlaws, Ochils, Pentlands, and the chain of hills running from the Pentlands by Tinto, Douglas, Corsincone ..and Loch Doon, into Ayrshire. -During this prolonged time of volcanic action there seem' to have. been many local disturbances of level over and above the general subsidence already referred to. - The most,widespread and important of these took place after the deposition of the Lower Old 'Red Sandstone. The whole physical geography of the gountry was changed j a wide interval passed, of which we have in the Edinburgh district no record, and when the next set of strata, those of the so-called Upper Old Red Sandstone, came to be laid down, the sandstones, conglomerates, and lavas of the former inland seas and lakes had been elevated into land. The Upper Old, Red Sandstone forms the base of the great Carboniferous System of central Scbtland. It consists of red, yellow, white; and greenish sandstonks, clays, congIomerates, and occasional limestones. At Edinburgh it forms the ground on which the city is built, from the Castle to Arthur's.Seat, whence it stretches southwards .along the west flank of the Braid and Pentland Hills, rising in the East and West Cairn Hills fa a height of 1839 and 1844 feet above the sea. In this region it has yielded very few fossils. Some plants occasionally occur in the sandstones, and minute crustacea (Lqberditiu) are found in some of the calcareous bands. Toward the end of the formation of these red strata, volcanic action, which would seem to have been dormant here since the extinction of the Pentland volcanoes, broke out anew. The site of Edinburgh was covered by' the ejections of at least one volcanic vent, the rocks of Arthur's Seat, Calton Hill, and Craiglockhart Hill being some of the remaining fragments of these ejections. Probably the rock on which Edinburgh Castle stands may represent the site of one of these volcanic craters. From this time onward the @eat plain of the Lothians and Fife was dotted oveE with little active volcanoes, each throwing out a comparatively small amount of dust or lava and then dying out, while fresh successors appeared elsewhere. Above the red sandstone on which Edinburgh is built lies a great mass of white sandstone and black shales, which spread westward from the Castle and extend northward to Granton and Leith. . They seem to have been laid down in an estuary or in some inland sheet of fresh water to which the sea had occasional access. The plants include large conifers (like the huge trunks which have from time to time been exhumed from the sand- They abound in fossils.
Volume 11 Page 208
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