Page:A Treatise on Geology, volume 2.djvu/173

 CHAP. VIII. Derbyshire, Flintshire, and Mendip. In the same manner the veinstones vary; even the calcareous spar is crystallised with quite different planes in the mines of Aldstone and Derbyshire.

The limits of mining districts are often very decided. In the rich mining tract round Cross fell, dissected like a map by mineral veins, and worked with an enterprise worthy of all praise, no instance (we believe) has yet occurred of a single vein being traced to the western side of the mountain range, across the great Penine fault, so as to penetrate the slaty rocks that rise in the line of dislocation. The same fact is witnessed again, in almost precisely similar circumstances, in the Flintshire veins, which do not, in a single instance, enter the subjacent silurian rocks of the Moel Fammau range, which rises on the line of a great axis of movement. Numerous instances of this remarkable dependence of the occurrence of mineral veins, in limited portions of country definitely related to particular lines of disturbed strata, are well and familiarly known.

Ever since the analogy of mineral veins and rock dykes has been clearly perceived, and the dependence of these latter on disturbance of subterranean temperature recognised, the dependence of the occurrence of mineral veins on the general influence of heat has been continually more and more apparent. This appears to have been strongly felt by Boué and Humboldt; there are also passages in the writings of Von Buch which conduct to the same conclusion. M. Necker presented to the Geological Society, in 1832, an attempt to bring under general geological laws the relative position of metalliferous deposits with regard to the rock formations of which the crust of the earth is constructed. The doctrine of the sublimation of the metalliferous contents of veins from igneous matter occurred to the author, twelve