. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ground plan of greenstone dike traversing sandstone. Arran. In the Hebrides and other countries, the same masses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea cliffs, pro- longed downwards in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ground plan of greenstone dike traversing sandstone. Arran. In the Hebrides and other countries, the same masses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea cliffs, pro- longed downwards in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of Skye, is no less than 100 feet in width. Fig. Trap dividing and covering sandstone near Suishnish in Skye. (MacCulloch.) Every variety of trap-rock is sometimes found in dikes, as basalt, greenstone, felspar-porphyry, and trachyte. The amygdaloidal traps also occur, though more rarely, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruptions on the land may be showered into them from the air. Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length. In many cases trap at the edges or sides of a dike is less crystal- line or more earthy than in the centre, in consequence of the melted matter having cooled more rapidly by coming in contact with the cold sides of the fissure ; whereas, in the centre, where the matter of the dike is kept longer in a fluid or soft state, crystals are slowly formed. But I observed the converse of the above phenomena in Teneriffe, in the neighborhood of Santa Cruz, where a dike is seen cutting through horizontal beds of scoriae in the sea-cliff near the Barranco de Bufadero. It is vertical in its main direction, slightly flexuous, and about one foot thick. On each side are walls of com- pact basalt, but in the centre
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
