146 Dr. Berger o;/ the physical Structure 



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 reason- 

 ably induce us to suppose, that they were very near the main body 

 of the granite.* 



4. These granite veins, whether they are found at the surface or 

 at a greater or less depth, are in Devonshire and Cornwall, invari- 

 ably directed from north to south, which is a direction opposite to that 

 of the metallic veins, but quite conformable with the mode of de- 

 position 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 hare been found at a great depth in some of the mines in CornwaH, 

 such as Dolcoath, Iliiel Providence, Unci Gorland, &c. — It is worthy of remark, that 

 the prefix Huely which is given to the greater number of mines in Cornwall, is the same 

 as that by Avliicli they are dislinguislied on the opposite coast of the continent, in 

 Brittany. These veins are called /orfe* by the miners, and divided into two classes, those 

 of granite being called ^roK'OW, and those of porphyry c/raw. The following are ex- 

 amples of these two kinds : — 



a. Vein of granite, found at the depth of one hundred and sixtj -one fathoms 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 app<»arance, and o])alescent ; the plates of mica are few, as well as the crystals of 

 felsjjar ; 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. 



