1823.] Geology of Devon end Cornwall. 187 



been noticed, that these disintegrated tracts exhibit frequent 

 patches of various dimensions retaining their original compact- 

 ness. It would be a point of some geological interest to ascer- 

 tain, as accurately as circumstances permit, the extent to which 

 this disintegration actually obtains, and to enquire whether the 

 tracts in question exhibit any phenomena which might guide our 

 speculations as to its probable cause. That that cause is not to 

 be sought in the mere superficial action of weather and moisture, 

 we may, I think, argue from the great depth to which (as at 

 Carglaise) the disintegration is found to extend. I have heard it 

 suggested, that this state of granitic rocks is to be regarded 

 rather as the mode of their original formation than as the result 

 of causes acting subsequently. This hypothesis, however, seems 

 hardly reconcileable to the appearances presented by Carglaise. 

 Some may, perhaps, incline to view the phenomenon as analogous 

 to the disintegration which is known to obtain largely in crystal- 

 line rocks forming acknowledged dykes,* and to these it may 

 appear to countenance the theory which attributes to granite an 

 igneous origin. At all events, this species of decomposition 

 seems for the most part peculiar to rocks of a crystalline struc- 

 ture. In some cases, if my observation be correct, the granite 

 is traversed by porphyritic dykes, or elvans, similar to those 

 occurring in the slate. I noticed one (in company with Mr. 

 Buckland) in 1812 on the summit of Kitt Hill, near Callington. 

 In some cases too the saalbande, or wall of metalliferous veins 

 traversing the granite, appears to be of this class. At Bean 

 Mine, about one mile east of Roche, the surrounding country is 

 granite of the usual aspect; while the walls of the tin lode which 

 are of considerable thickness have all the character of an elvan 

 dyke, the paste of which is chiefly quartzose with a mixture of 

 mica, talcite, felspar much disintegrated, and crystallized shorl, 

 the tin occurring in veins with shorl towards the centre of this 

 elvan (the veins occasionally send out small lateral branches at 

 various angles), the elvan itself is said to dip three feet in the 

 fathom, and in its general character much resembles some of 

 those found to the east of St. Agnes. The same inclusion (if I 

 may so term it) of a metallic vein by a rock differing from the 

 surrounding granite I recollect to have noticed also at New- 

 bridge, and in the Gwennap cluster. It would be desirable to 



to a much greater distance from the top of the hill, would have been taken to be granite, 

 as ihe surface has every appearance of a growan soil intermixed with granite stones and 

 rocks plentifully scattered about. The same has been observed on the sides of Cam 

 Marth and other granite hills bordering on the Killas country." We were assured that 

 at Newbridge they had sunk above the granite country in a mass of granitic rubble with 

 occasional blocks of killas intermixed to the depth of nearly 30 yards, and above the 

 killas country in a rubble of that rock with a small admixture of killas to about half that 

 depth. 



• See Mr. Henslow's Account of Anglesea. (Cambridge Philosophical Transac- 

 tions, Tart II.) 



