Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/188

 176 saalbande is in general of a different nature from the substances to which it is contiguous: it is sometimes composed of clay, sometimes of amianthus, of mica in layers, &c. It is not found in all veins, and those from which it is absent are said to be adherent. This is, I believe, generally the case in Cornwall.

Vein-stones are the different stony substances with which the ore is intermixed, and which as a whole constitute the vein. It is these vein-stones which I now propose more particularly to consider.

Werner is of opinion that in the same vein the parts of the vein-stone nearest to the saalbande are the oldest, those in the middle the most modern, and the intermediate parts of a middle age. But whatever may be the age of these different parts of the vein, they are all necessarily posterior to the rock of the mountain in which the vein occurs, and in the present case, to the granite and the grauwacke, the only two formations of any extent that are met with in Cornwall and Devonshire.

As the same vein-stones are found in different formations of veins, we must therefore admit, that the same rocks have been held in solution at different periods.

Massive quartz sometimes forms alone the greatest part of the vein-stone of certain veins. This is the case at Kithill near Callington: I believe that here the quartz is united to the granite without any saalbande intervening.

In the tin mines between St. Just and Cape Cornwall, quartz is united with black, massive, and radiated tourmaline. The quartz exists not only in the form of veins, but also in blocks. According to Bergman, there is in the mountain of Nasa, a block of quartz several hundred yards broad, and double that dimension in length.