Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/192

 180 their matrices. Before closing this memoir, I shall venture to offer some conjectures on the causes to which some of the great phenomena in the physical structure of Cornwall may be attributed, and on the epochs of their occurrence, more particularly with respect to the formation of the veins and the cross-courses.

It appears to me, that the force which produced the fissures that now contain the veins must have acted upon the northern and southern slope of the chain: that at the time when the waters fell into the British Channel on the one side, and into the Bristol Channel on the other, the granite ridge, as well as the upper part of the strata of grauwacke, being left exposed and in a state of softness, must, when left to their own weight, have yielded on the sides where they were no longer supported, and that owing to this cause, fissures, or empty places, were formed on each slope, parallel to the length and to the direction of the chain, that is to say, from east to west; and that these spaces, being afterwards filled up with different materials held in solution, formed the veins such as we now see them.

But the body of water still serving as a support to the strata at the base, while they were deprived of it in the upper parts, it necessarily followed that the fissures were confined to the parts of the chain which were first left exposed; and we find, in fact, that the greater number of the mines are situated on a line which is a little below that of the junction of the granite and the grauwacke.

Farther, there must have been at an after period some great convulsion, which produced the falling down or hanging of the chain towards the west; the formation of the cross-courses, and consequently the breaking across of the true veins ; and lastly, the dislocation of some portion of the chain, the fragments of which carried