A dyke is a vertical or steeply inclined wall of igneous rock which was forced up into a fissure when molten and there consolidated. Dykes of a certain kind may actually be seen in the making, as when the lava column of a volcano bursts its way through fissures in the cone. The ordinary dyke is formed in fissures which traverse stratified rocks, breaking across the bedding-planes and usually approximating a vertical position, though sometimes it cuts through older and already consolidated igneous rocks. In thickness dykes vary from less than a foot to a hundred feet or more, and pursue nearly straight courses, it may be for many miles. The rock of a dyke has usually a compact texture, having cooled more slowly than the volcanic masses, though the edges, chilled by contact with the walls of the fissure, may be glassy. If the rock displays columnar jointing, the prisms are horizontal, normal to the cooling surfaces.

Parallel dykes, Cinnabar Mountain, Montana. The left hand dyke forms the distant peak.

Fig. 210. - Parallel dykes, Cinnabar Mountain, Montana. (U. S. G. S.) The left-hand dyke forms the distant peak.

Dykes may be so numerous as to form a regular network of intersecting walls, just as we have found to be the case in faults and fissures.

The commonest rocks in dykes are basalt, quartz porphyry, andesite, and diabase.

When denudation has so far cut away the surface of the ground as to expose the dyke, the form which the latter takes will depend upon the relative destructibility of the igneous rock and the enclosing strata. If the latter wear away more rapidly, the dyke will be left standing above the surface like a wall (Fig. 210); but if the igneous mass be disintegrated more rapidly than the strata, a trench will mark the line of the dyke.

Dyke trenched by weathering faster than country rock. (U. S. G. S}.

Fig. 211. - Dyke trenched by weathering faster than country rock. (U. S. G. S}.

Dykes are common and conspicuous objects in the Connecticut valley and in the sandstone belt which runs, with interruptions, from the Hudson River to North Carolina.

Intrusive Veins are smaller and more irregular, frequently branching fissures which have been filled with an igneous magma; they may be only a few inches in thickness, and may often be traced to the mass which gave them off. The nature of the rock in a vein may be much modified by material derived from the walls. This vein rock is often so coarsely crystalline, as in pegmatite veins, that it has been suggested that it could not have solidified from fusion, but was deposited from solution in superheated waters.

Veins of granite in cliff, near Gunnison, Col. (U. S. G. S).

Fig. 212. - Veins of granite in cliff, near Gunnison, Col. (U. S. G. S).