. Elements of geology. Geology. 104 LYELL'S ELEMENTS OF GEOLOGY. Trap Dikes and Veins. In the islands of Arran, Sky, and other parts of Scotland, where sandstone, conglomerate, and other hard rocks are tra- versed by dikes of trap, the converse of the above phenomenon is seen. The dike having decomposed more rapidly;- than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast, as represented in the annexed view. (Fig. 90.) In these instances the greenstone of the dike is usu- Fig, 90. ally more tough and hard than the r—
. Elements of geology. Geology. 104 LYELL'S ELEMENTS OF GEOLOGY. Trap Dikes and Veins. In the islands of Arran, Sky, and other parts of Scotland, where sandstone, conglomerate, and other hard rocks are tra- versed by dikes of trap, the converse of the above phenomenon is seen. The dike having decomposed more rapidly;- than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast, as represented in the annexed view. (Fig. 90.) In these instances the greenstone of the dike is usu- Fig, 90. ally more tough and hard than the r— ^: sandstone ; but chemical action, and chiefly the oxidation of the iron, has given rise to the more ra- pid decay. There is yet another case, by no means uncommon in Arran and other parts of Scotland, where the strata in contact with the dike, and for a certain distance from it, have been hardened, so as to resist the action of the weather more than the dike itself, or the surrounding rocks. When this happens, two parallel walls of indurated strata are seen protruding above the general level of the country, and following the course of the dike. As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifur- cating and ramifying, and sometimes they are so tortuous as to be called veins, though this is more common in granite than in trap. The accompanying sketch (Fig. 91.) by Dr. MacCulloch repre- sents part of a sea-cliff*in Argyleshire, where an overlying mass of trap, sends out some veins which terminate downwards. Another trap vein, a a, cuts through both the limestone, c, and the trap, b. In Fig. 92. a ground plan is given of a ramifying dike of greenstone, which I observed cutting through sandstone on the beach near Kildonan Castle, in Arran. The larger branch varies from five to seven feet in width, which will afford a scale of measurement for the whole. In the Hebrides and other countries the same masses o
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