Dyke

Dyke. 1. In Gcoloyy, a vertical or highly inclined sheet or vein of intrusive igneous rock, from an inch or two to 60 or 70 feet thick, extending for a few yards or for many miles, the thicker ones being generally more persistent. They may consist of granite, pegmatite, or curite, but are more typically basaltic, basalt, dolerite, diabase, and diorite commonly occurring in this form. They often exhibit prismatic jointing, the prisms being horizontal, at right angles, that is, to the surfaces of cooling of the dyke, and their outer surface i3 often glossy from a layer of tachylite (basaltic glass). Though they serve as girders binding together the loose tuffs of volcanic cones through which they are thrust, as in the Yal del Bove, Etna, in Vesuvius, in Santorin, and at Funchal, they often traverse non-volcanic rocks for miles in straight lines, sometimes in parallel lines. Their material being often harder than the rock they traverse, denudation has left them standing out like walls (Scotic'v, dykes), as is the case with the diabasic Great Whin Sill of the North of England, along part of which the Roman wall was carried. In other cases the dyke has been disintegrated first, leaving deep ditch-like ravines. Dykes frequently graduate into veins or sheets. They alter the rock they traverse, indurating clay into porcellanite and converting chalk into crystalline marble, as in Rathlin Island, or coal into soot, or at a greater distance into graphite, as at Cumnock in Ayrshire.

2. In Engineering, means an embankment of earthwork generally erected to ward off water from a tract of land. Dykes are necessary in low-lying countries like Holland, where inroads are frequently attempted by the sea. They must be well and strongly built, and repairs must be effected as soon as any damage is done. The bursting of dykes has caused immense loss of life and property in Holland, Hungary, China, and elsewhere.