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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.
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150 EDINBURGH PAST AND PRESENT. stone quarries of Craigleith and Granton), several species of L~idodmdronan d some. characteristic ferns, particularly the Sphojte~isa# nis and Adianfi2e.r Lindserefomrj, and other forms. Some of the shales are charged with enfomustram (Lqkdifia Smf&Bura‘zgdmk) and the scales, teeth, bones, and cop’rolites of ganoid fishes such as PaZmnrjcus, Ewynofus, Megaiichfhys, and Rhizodus, The marine intercalations are indicated by the presence of such forms as Spirorbis, LinguZa, Schizodus, Myaiina, BelZ~rojhon, and Orfhoceras. Some of the individual strata of this series are well known, either from their geological or industrial interest. Thus the Burdiehouse limestone, long quarried about four miles south from Edinburgh, is formed apparently from the aggregation of the cases of little crustacea, chiefly of the genus Lcperdifia, and has yielded a large number of well-preserved plants and fishes. The ironstone nodules of the Wardie beach Are likewise noted for their fossil remains. The sandstones of Craigleith, Granton, Redhall, Hunibie, and Binny have supplied the best building-stones in this part of Scotland. More recently some of the highly carbonaceous shales have been turned to account as profitable sources of mineral oil. Next in order comes the Carboniferous Limestone series, which in the Mid-Lothian coal-field attains a thickness of 1220 feet. It consists chiefly of sandstone and shales, with some bands of marine limestone and many valuable skams of coal. Like the rest of this formation in Scotland, it was formed in wide shallow lagoons, which at one time were covered with vegetation as the mangrove swamps of the tropics now are, an’d at another, owing to the subsidence of the ground, were submerged beneath salt water in which characteristic marine forms of life abounded. The former condition is represented by the coal-seams which consist of the compressed and mineralised vegetation that grew upon the spot; the latter by the seams of limestone, full of crinoids, corals, and brachiopods. Spines of various shark-like fishes, as well as scales and teeth of others like the bony pike of the North American lakes, occur actually on some of the coal-seams, one seam in particular being marked by such an abundance of fish remains as to form a kind of ‘bone-bed.’ The plants include the usual Carboniferous genera, as Eepidudmdron, [email protected]&, Cdamites, Cordazh, Sphenojteris, Pecopierzk, etc. The lower limestones are thickest. They may be seen at Gilmerton, and, still better, at the great quarries of Cousland, Darcy, and Crichton. Their fossils agree with those of the true ‘Mountain Limestone’ bf the centre and northern English counties. The coal-seams form what is known locally as the ‘ Edgecoal ’ series, from the fact that owing to a large dislocation which
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