34S Mlncralogical Observations on Cornwall. [Nov. 



of an old hay-stack partly cut away with a hay-knife. This 

 brownish yellow colour is owing to the great preponderance of 

 felspar in this rock. 



The Logan rock, about three miles south from the Land's 

 End, consists of an immense block of the same kind of granite, 

 so poized upon a point as to be moveable by the force of a man. 

 Veins of schorl are said to exist near it, but 1 did not see them. 



The primitive granite of Cornwall appears to constitute a tract 

 of no great height nor breadth, extending from the Land's End 

 to the extremity of Dartmoor in Devon, and therefore running 

 in a direction not very far from south-west and north-east. I 

 did not indeed trace it the whole of this way, but I crossed it 

 in three several places, at a considerable distance from each 

 other, and therefore have reason to think it highly probable that 

 the rock continues all this way (a length of about 120 miles). 

 The Scilly Islands are likewise granite, and are doubtless a coiir 

 tinuation of the same range. This granite range terminates in 

 the si a at both extremities. It rises gradually all the way from 

 the Land's End to Dartmoor, where it constitutes mountains, 

 some of which, as we barn from the trigonometrical survey, are 

 1549 feet above the level of the sea* 



This granite range of ground does not exceed three miles in 

 breadth (and indeed at Redruth appears narrower) in Cornwall, 

 but in Devonshire its breadth is considerably greater. About 

 half way between Bodmin and Launccston, you cross another 

 aranite tract. It would seem that it branches off from the east 

 and west granitic range nearly at right angles ; or perhaps the 

 granitic range becomes wider at this place than usual. It 

 appealed to go north some miles beyond the high road from 

 Bodmin to Launccston, and constituted all the way a tract much 

 more elevated than the rest of the country, and rising in some 

 places so as to form mountains. 



Clay slate rests upon this granite range, both on the north and 

 south side This clay slate on the south side dips south, and on 

 the north side, north, showing evidently that it lies over the 

 granite. On the west side of the north and south granite range 

 it dips west ; and at the east side, towards Launccston, it dips 

 east. When we examine it at a considerable distance from the 

 granite range, it appears horizontal. These observations, which 

 I made in a variety of places, leave no doubt that this extensive 

 bed of clay slate every where covers the granite, and is nowhere 

 covered by it. It is likely that the granite rock extends 

 very widely, and may even constitute the basis of the county ; 



» According to that survey, Brown Willy fs 1368 feet, and Rippintor 1.5!') 

 feel, above the level of ihe sea. According to Ur Berger, Cape Cornwall 

 near Ihe Land's Esid is 229 feet, aud Carnbiae (a hill at Redruth) 697 feet, 

 above the level of tilt sea. 



