186 Rev. J.J. Coin/beare c»i the [March, 



I. Granite. — The character of this rock is, so far as I am 

 acquainted with it, remarkably uniform through the whole extent 

 of its range. That of Waterloo Bridge may serve as a specimen.* 

 Near the "points of junction with the incumbent slates, it occa- 

 sionally becomes small grained, and of a redder hue, resembling 

 the granite of veins. Examples may be found at a junction near 

 Ivy Bridge, at Buckland on the Moor, above Belstone (near 

 Okehampton), and near Bovey Tracey at the spot which produced 

 the tine specimens of tourmaline and apatite. As noticed by 

 Mr. Sedgwick in these cases, there appears to be a diminution, 

 sometimes a total loss, of the mica. The predominant variety of 

 granite contains also in many places patches of a smaller grain, 

 generally of a form more or less spherical. These difter so much 

 in their aspect from the general mass, that, by a casual observer, 

 they might be taken for imbedded portions of another rock ; a 

 more accurate inspection will soon show that this is not the 

 case, even in those instances in which (as near St. Just) the 

 predominance of dark coloured mica or chlorite in these patches 

 gives them much the aspect of a kind of gneiss. Other instances 

 may be found near the Land's End, near Moreton Ilampstead, 

 above Henoch, and on the road leading from Bovey Tracey to 

 the depot of tourmalines, &c. already mentioned.f 



The disintegration of granite in situ, as exhibited on a large 

 scale at the porcelain clay pits in St. Stephen's, and tiie open 

 mine of Carglaise,1: near St. Austel, has been often noticed. 

 Another large tract of the same character will be found on Dart- 

 moor, in the neighbourhood of a hamlet termed (from the nature 

 of the soil thus produced) Sandy Park. Other partial instances 

 occur, and the granitic tors, the formation of which has been so 

 ably illustrated by Dr. Macculloch, afford abundant proof that 

 the action of the causes which produced this phenomenon has 

 been at some time or other nearly universal.^ It has, I believe, 



* See Mr. Sedgwick's paper (p. 10). It may here be noticed that the constituent 

 minerals of granite are seldom found in die west (as in many similar tracts) distinctly 

 or saparatcly crystallized. The imbedded crystals of felspar have been noticed by Mr. S. 

 and others. I found it crystallized in rhombs in small cavities on St. Michael's Mount, 

 and in small rhombs and in larger crystals of a more complicated form at the tourmaline 

 pit near Bovey Tracey. I have also from Cornwall, but without the exact locality, two 

 specimens of perfectly crystallized mica, the one a rhomboidal, the other an hexagonal 

 tablet. Both are in a small grained reddish granite (possibly an elvan), the aspect of 

 which docs not resemble that of the specimens brought from Scilly by Air. Majendie. 

 I may add, that on St. Michael's Mount, I observed in a highly felspathic portion of 

 the granite insulated crystals of felspar rendered as tender as the softest clay by some 

 process of decomposition which had not affected the imbedding mass. 



t Mr. Sedgwick mentions the same phenomenon as observable near Castle Trereen 

 (the Logging flock) : to his remarks I would refer the student for much fuller inform- 

 ation. 



+ It is, perhaps, hardly worth while to state, that the finest porcelain clay which I 

 obtained in Cornwall was from a vein in Carglaise. It occurred in very small quan- 

 tities. 



^ Mr. Taylor (Report, p. 1 ) affords an interesting illustration of one result of this 

 process. " If the ground, 80 fathoms south-east of Cam-brae, and at Wheal Druid, 

 had not been penetrated by the mines, it is very possible that the whole of it extending 



