degree of richness, for instance, if a vein was in grauwacke, it would be impoverished by entering the granite, and vice versa.
It was formerly conceived, that the veins of copper belonged exclusively to the grauwacke, and that those of tin were chiefly confined to the granite; but there are now several copper veins worked in the primitive rock: it is however true, that they are near its junction with the grauwacke.[1]
Cross courses are veins of marl or clay which intersect the true veins in Cornwall nearly at right angles to their direction, that is to say, which run from north to south. The most considerable of these cross courses extends from sea to sea; it passes directly through the meridian of St. Agnes, leaves in the middle of its course the parish of Stythians, three quarters of a mile to the west, and terminates on the south in the neighbourhood of Pedn-Boar-Point: it varies from a few inches to some feet in thickness: the depth to which it penetrates is still undetermined. It not only intersects all the true veins, but it has thrown the western portion of those veins some fathoms to the north of the corresponding portion on the east side of it. These cross courses are evidently of posterior formation
- ↑ Mr. Kirwan has endeavoured to explain, why ores are found less frequently in granite than in all other rock formations. He says, “ Hence we see why metallic veins seldom occur in granitic mountains or those of jasper, and the harder stones, as their texture is too close to permit the percolation of water, at least in sufficient plenty, and because their rifts were previously occupied and filled with stony masses, as being more soluble, and therefore soonest conveyed into them; thus silex sufficiently comminuted is soluble in about one thousand times its weight of water, or even less, whereas metallic substances require much more; but if the granitic stones are in a state of decomposition, as in the lower mountains they often are in Cornwall, &c. there they may be metalliferous. On the other hand, gneiss and schistose mica, argillaceous porphyry, and argillites being much softer, are the principal abodes of metallic ores.” Geological Essays, p. 412.
