VEINS, in geology, are crevices, more or less vertical, caused by the contraction during drying or metamorphoses, or by the mechanical disturbance of a rock, which have been filled by materials different from the body of the rock. Veins containing sub tames that have been injected in a state of fusion from heat, have bad their origin in some internal force; while those filled with mineral deposits may or may not be con nected with upheaval. Granitic and trappean veins differ from dykes chiefly in the ,' greater size of the latter. They produce similar changes in the rocks which they pene trate, indurating clays and sandstones, and converting limestones into marble, or giving them a compact texture like hornstone. Granite veins are generally more sinuous in their course than those of trap. One set of veins often intersects another, having beed produced at a later period; and the two sets generally differ in color, grain, and even mineral composition. Granite generally assumes a fine grain, and is even differ cut in composition in the veins it sends into the adjoining rocks. Mineral veins are tilled with different kinds of crystalline minerals. Quartz and calcite arc the most corn moo of these substances; but frequently several different minerals occur in the same vein, some of these being metallic ores. Veins of the same age are filled with the same
metals, and generally maintain a general parallelism of direction. Thus, the tin and copper veins of Cornwall run nearly e. and w.; while the lead veins run n. and south. Three kinds of veins are distinguished by the miners—rake, pipe, and fiat veins. The rake veins are simple crevices, crossing all the rocks of a series, generally highly inclined, and apparently formed from the contraction of the rock. The two originally opposite surfaces may retain their relative positions, only separated by the interposed contents of the veins; or a fault may place the originally contiguous surfaces at differ ent levels; and in such a case, the intervening spaces between the walls of the vein are irregular, sometimes narrowing so that the walls are in contact, and then widening out, and forminglarge cavities containing ores. The pipe veins arc irregular cavities, filled with minerals, and without any apparent connection with faults in the strata. Flat veins have a general direction corresponding with that of the stratification, and are connected sometimes with rake veins, and sometimes with pipe veins. The manner of working the minerals contained in veins is explained in the article 11ccrso (q.v.).