Dykes and Veins. Igneous rocks which rise in even-sided, more or less vertical, wall-like sheets are called dykes, from the common Scotch word for a wall. The term vein is applied to the more irregular, winding, branching, and generally smaller intrusions. Dykes and veins are of common occurrence in the cones of existing volcanoes, where they appear to have been injected chiefly from below. Occasionally, however, the rents occupied by dykes would seem to have been filled from above by an overflowing stream of lava. The crystalline rocks of which dykes are composed are of various kinds. In Scotland, where dykes are abundantly developed, the rock is chiefly some variety of basalt-rock. These basalt-dykes vary in thickness from a foot or less up to 100 feet and more; and in length, from apparently only a few yards up to many miles. Sometimes they cut across rocks which have yielded more readily than themselves to the denuding agents, and hence they form prominent features in a landscape, stretching like great wall-like ramparts across low-lying undulating tracts; when, on the other hand, they traverse strata which are less readily eroded than themselves, they frequently form deep trench-like hollows. Sometimes they have come up along lines of faults; but more frequently they appear simply to occupy great rents which are not accompanied by any vertical displacement of strata. They generally run in approximately straight or gently undulating lines; but occasionally they follow a more zigzag course. The rock of a dyke is usually jointed at right angles to its direction, and now and again this jointing gives rise to fine prismatic columns. In the centre of the dyke the rock is more markedly crystalline than towards the sides, where it is often compact, and its point of junction with the walls of the rent in which it lies are not infrequently coated with a skin of volcanic glass. Vesicular cavities frequently appear in the centre of a dyke, and finer pores are often distributed through the rock near the sides. As a rule, the strata are not much affected at their junction with a dyke—the alteration seldom extending beyond a foot or two. When the dyke, however, is very thick, say 100 feet or so, the adjoining rocks are often considerably baked—limestones being rendered crystalline, while coals are converted into a kind of coke, shales are porcelainised, and sandstones changed into quartzite. Sometimes, too, the strata are much smashed and jumbled, and their fragments inclosed in the marginal areas of the intrusive rock.
In regions where fissure-eruptions have taken place, the dykes are often branching and irregularly ramifying—the rocks being traversed by a perfect network of dykes and veins, anastomosing with and crossing each other at all angles. Good examples occur in the western islands of Scotland, the Faroe Islands, and Iceland. Veins are frequently very numerous in the neighbourhood of great masses of granite, from which, indeed, they proceed. It is remarkable that the rock of such veins is frequently finer grained than the granite from which it comes, and often passes into quartz-porphyry or Felsite (q.v.). Granite itself is also very commonly traversed by peculiar dykes and veins, some of which are more coarsely crystalline, while others are finer grained, than the granite itself. These veins are so closely welded into the granite, their crystals indenting the surrounding rock, that it is obvious they were formed at a time when the granite was only partially consolidated. It seems probable that they were injected before the granite had quite solidified. They are known as 'contemporaneous' or 'segregation' veins; but their precise mode of formation is still very obscure. Occasionally dykes of fragmental matter occur, as in the Sidlaw Hills, the hills of Ayrshire, and the Cheviots in Scotland. They have been observed also in the Canary Islands. Such dykes vary in width from a foot or two up to many yards. They seem only to occur in association with other volcanic rocks, and generally to indicate the proximity of some volcanic vent. The fragmental materials are angular, and form a breccia or agglomerate of volcanic rocks alone, or of these and various derivative rocks, hence they are termed agglomerate dykes. See IGNEOUS ROCKS.