Page:A Treatise on Geology, volume 2.djvu/113

 CHAP. VII.

T. The great mass of basalt in Teesdale. d. A straight dyke passing East 20 North. d′. Another, passing generally to the South of East.

perfectly straight line, across the Durham coal-fields, twenty miles, in a direction E.N.E.; the other, starting from the same point (near Middleton in Teesdale), extends into the eastern part of Yorkshire, nearly reaching Robin Hood's Bay, a distance of seventy miles, in an E.S.E. direction.

In some districts, rock dykes are wonderfully numerous. Forty-four trap dykes of various kinds were carefully noticed and measured by the author of these remarks, in a few miles of the coast of the Island of Arran, between Brodick and Lamlash. They abound no less on the western side of the same island at Tormore.

Veins.—One of the most interesting forms of occurrence of igneous rocks is that of veins, which penetrate and ramify irregularly in the fissures of the neighbouring rocks. These veins sometimes appear insulated in the midst of rocks more or less different from them in composition, except at the common surfaces, where the substance of the vein and the in closing rock are intimately united by intermediate characters of mineral composition or indistinguishable blending of the parts. In this manner granite frequently incloses parts in which hornblende, or mica, are particularly abundant or remarkably deficient; the redundancy and defect being equally referrible to circumstances which operated during the crystallisation of the stone. To such spherical, nodular, or elongated parts of a rock, the title of