of Devonshire and Cornwall. 143 



it Is common grauwacke, with quartz,* and at Its junction with the 

 granite, it is traversed by veins of this rock, similar to those I have 

 aheady mentioned in the valley of the Erme in Dartmoor, f The- 

 southern side of the mount is nearly precipitous, and is composed 

 from top to bottom of a granite split into irregular masses ; at the 

 bottom is a heap of large blocks, among which, I thought I observed 

 some indications of copper ore, on their surface. Sometimes the 

 felspar and sometimes the quartz predominates in this granite; when 

 it is the quartz, it gives the rock a vitreous appearance : it contains 

 also black tourmaline, and pinite is also said to have been found 

 in it. 



Admitting however that the mass of granite of St. Michael's 

 Mount was detached from the land, before the grauwacke was de- 

 posited upon it, and conformable with it, the grauwacke could only 

 have rested on the northern face, or that which is the least abrupt, 

 as the southern face is almost perpendicular, which is shewn by the 

 great depth of the sea at the bottom of the mount on that side. But 

 without having recource to this hypothesis, to explain so partial a 

 fact, it would perhaps be more reasonable to admit, that the epoch 

 of the separation and transportation of St. Michael's Mount has been 

 posterior to the deposition of the grauwacke which has remained 

 adhering to the detached mass of granite ; and that in settling it 

 has taken such a degree of inclination, that the strata of grauwacke 

 on the south have been completely concealed, and only exposed to 

 view on the northern side. I should not have dwelt so long upon 



* The large rocks lying on the bar between Marazion and the Mount, are also com- 

 mon grauwacke. 



+ Mr. Playfair has described this appearance, with a degree of precision propor- 

 tionate to the importance he attaches to facts of this sort. Illustrations of the Huttonian 

 Theory, p. 318. 



