Page:A Treatise on Geology, volume 2.djvu/112
98 CHAP VII.
A TREATISE ON GEOLOGY.
Wrekin, the Lawley, Caer-Caradoc, &c.; and, among existing volcanos on a greater scale, the linear volcanos, to which Von Buch was the first to direct attention. Great fissures, such as here alluded to, may be extremely irregular; the strata through which they break may be thrown into great confusion; their parts may be disjoined and separated by cavities. Into these irregular hollows the fused matter sometimes has been forced; and not infrequently large and small portions of the broken strata are inclosed in the midst of the igneous rock; while sometimes portions of the latter have flowed into cavities in the stratified masses, from which it is difficult to trace their connection with the main stream.
Such phenomena may be well studied in Salisbury Craigs, and other localities near Edinburgh; in Teesdale; the Caradoc Hill, &c.
Dykes.—A still more common form of appearance among igneous rocks is what is called a dyke, which agrees with the general description of similar rocks occupying a fissure; nor in some cases is there any distinction. But dykes, when seen in perfection, as in the Island of Arran, the coal-field of Durham and Newcastle, the limestone of Teesdale, the lias near Stokesley, the silurian rocks of Shropshire, or the slates of Snowdonia, present characters of greater symmetry, and claim a somewhat different origin. The fissures which inclose these trap dykes present often no trace of violent movement of the strata, which, on the contrary, sometimes appear level and undisturbed on both sides; these sides are remarkably parallel, plane, and either vertical, or slightly inclined, so that the inclosed mass of rock looks like a continuous wall. On the surface the dyke lies usually in a straight line from a few hundred yards to ten, twenty, and more miles in length.
Archdeacon Verschoyle has described several trap dykes which range on the coast of Mayo and Sligo: one of them extends altogether, in an east and west direction, sixty or seventy miles. One of the dykes, which is represented in the diagram No. 83., continues in a