As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifurcating 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. 441.]) by Dr. MacCulloch represents part of a sea-cliff in Argyleshire, where an overlying mass of trap, b, sends out some veins which terminate downwards. Another trap vein, a a, cuts through both the limestone, c, and the trap, b.
In [fig. 442.], 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 5 to 7 feet in width, which will afford a scale of measurement for the whole.
Fig. 442.
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, prolonged 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. 443.
