230 ON GRANITE. 



metallic veins. The granite veins frequently interfecl; the me, 

 tallic, and are remarkable for producing fhifts in them, and 

 for throwing them out of their natural direction. The mine* 

 ral veins, particularly thofe that yield copper and tin, run 

 nearly from eaft to weft, having the fame direction with the 

 beds of the rock itfelf, which is very hard fchiftus. Thegra- 

 nite lodes, as alfo thofe of porphyry, called elvan, in Corn- 

 wall, are at right angles nearly to the former ; and it is re- 

 marked that they generally heave the mineral veins, but that 

 the mineral veins feldom or never heave the crofs veins. In 

 this country, therefore, veins of granite and porphyry are pof- 

 terior in formation to the metallic veins. Thefe veins of gra- 

 nite may perhaps be conneded with the great granitic mafs 

 that runs longitudinally through Cornwall, from Dartmoor to 

 the Land's End. This much is certain, that their directions 

 in general are fuch, that if produced, they would interfecl that 

 Ifluing like roots mafs, nearly at right angles." Further, at p. 317. he re- 

 chad's Mount mar ^ s > " The laft inftance I have to mention from my own 

 into the fchiftus, obfervation, is at St. Michael's Mount in Cornwall. That 

 mount is intirely of granite, thruft up from under a very hard 

 micaceous fchiftus, which furrounds it on all fides ; at the bafe 

 of it a great number of veins run off from the granite, and 

 fpread themfelves like fo many roots fixed into the fchiftus : 

 they are feen at low water. In the fmaller veins, the granite 

 is of very minute, though diftinft parts ; in the larger, it is 

 j more highly cryftallized, and is undiftinguifhable from the 

 mafs of the hill." I agree with Profeffor Playfair, in believing 

 that the Cornifh veins may ftand in connection with the gra-r 

 nite of the country, but fhould this be proved, the Cornifh 

 referable to the granite muft then be referred to the Greifenftein or Sienite 

 tion.' te f ° rma * f° rmation - The appearances obferved at St. Michael's Mount 

 demonftrate, that it belongs to one of the formations I have 

 juft mentioned ; and I have no doubt that if Profeffor Playfair 

 had examined the fituation of the primitive flate, with re- 

 gard to the granite, he would have found it covered by the 

 granite. 

 Other granite Again at p. 316, Mr. Playfair informs us, "that another 



vans in Ga o- f er ; es f granite veins is found in Galloway, which were firft: 

 difcovered by Dr. Hutton, and his friend Mr. Clerk, and af- 

 terwards more fully explored by Sir James Hall and Mr. 

 Douglas, the prefent Earl of Selkirk. The two laft traced 



3 the 



