3. In those places which have been thought to prove that granite veins were formed in the same way as metallic veins, they have always been at a considerable depth, such at least, as might reasonably induce us to suppose, that they were very near the main body of the granite.[1]
4. These granite veins, whether they are found at the surface or at a greater or less depth, are in Devonshire and Cornwall, invariably directed from north to south, which is a direction opposite to that of the metallic veins, but quite conformable with the mode of deposition of the grauwacke on the mass of granite which forms the low mountain chain of Cornwall.
5. These subterraneous granite veins are rarely metalliferous, but when they do contain a metal, it is always tin, which is known to belong to the oldest formation, and which sometimes forms one of the component parts of the granite.
- ↑ Granite veins have been found at a great depth in some of the mines in Cornwall, such as Dolcoath, Huel Providence, Huel Gorland, &c.─It is worthy of remark, that the prefix Huel, which is given to the greater number of mines in Cornwall, is the same as that by which they are distinguished on the opposite coast of the continent, in Brittany. These veins are called lodes by the miners, and divided into two classes, those of granite being called growan, and those of porphyry elvan. The following are examples of these two kinds:─
a. Vein of granite, found at the depth of one hundred and sixty-one fathom in the mine of Dolcoath. This granite ought to be called porphyritic, as the particles which compose it are so far separated from each other, that they may be said to be imbedded in a base of common felspar, which is in a state of decomposition. The quartz is of a bluish appearance, and opalescent; the plates of mica are few, as well as the crystals of felspar; these last are very well defined.
b. Vein of porphyry in the mine of Huel Providence. This rock is more compact, or more sound than the preceding; the base is a felspar, of a dirty grey colour, in which are imbedded small crystals of quartz, mica, and felspar of an opaque white colour.
In the Journal des Mines, No. xviii. p. 84, there are some observations on the causes which may have produced these alterations in the texture of granite.
