Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 612 TRAP DIKES AND VEINS. [Ch. XXIX. wider after the lava on each side had consolidated, and the additional melted matter poured into the middle space maj have cooled more rapidly than that at the sides. In the ancient part of Vesuvius, called Somma, a thin band of half-vitreous lava is found at the edge of some dikes. At the junc- tion of greenstone dikes with limestone, a sahlband, or selvage, of serpentine is occ
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 612 TRAP DIKES AND VEINS. [Ch. XXIX. wider after the lava on each side had consolidated, and the additional melted matter poured into the middle space maj have cooled more rapidly than that at the sides. In the ancient part of Vesuvius, called Somma, a thin band of half-vitreous lava is found at the edge of some dikes. At the junc- tion of greenstone dikes with limestone, a sahlband, or selvage, of serpentine is occasionally observed. On the left shore of the fiord of Christiania, in Norway, I examined, in company with Professor Keil- hau, a remarkable dike of syenitic greenstone, which is traced through Silurian strata, until at length, in the promontory of Nsesodden, it enters mica-schist. Fig. 682 represents a ground plan, where the dike appears 8 paces in width. In the middle it is highly crystalline and granitiform, of a purplish color, and containing a few crystals of mica, and strongly contrasted with the whitish mica-schist, between which and the syenitic rock there is usually on each side a distinct black band, 18 inches wide, of dark greenstone. When first seen, these bands have the appearance of two accompanying dikes; yet they are, in fact, only the different form which the syenitic materials have assumed where near to or in contact with the mica-schist. At one point, a, one of the sahlbands terminates for a space; but near this there is a large detached block, b, having a gneiss-like struc- ture, consisting of hornblende and felspar, which is included in the midst of the dike. Eound this a smaller encircling zone is seen, of dark basalt, or fine-grained greenstone, nearly corresponding to the larger ones which border the dike, but only 1 inch wide. It seems, therefore, evident that the fragment, b, has acted on the matter of the dike, probably by causing it to cool more rapid
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