Junction of the Granite and the KilLas Rocks in Corn'wall. 243 



composed granite, in which numberless double six-sided quartz 

 pyramids are imbedded : the contours of the crystals of felspar 

 may be seen very easily in the greater part of this granite ; it 

 must therefore be a granite decomposed in situ, and not de- 

 posited as an alluvium. The China-stone which is raised on 

 the western side of Tregonning Hill near Helston, is a granite 

 in which the decomposition has proceeded no further than to 

 render the whole mass friable; only one part of the felspar is 

 changed into clay. 



IX. St. Michael's Mount. — St. Michael's Mount is a small 

 island nearly three hundred feet high, situated in the bay, at a 

 distance of about three miles from Penzance; the greater part 

 of it consists of granite ; the killas appears only on the northern 

 side of the island, and is greenstone where it comes in contact 

 with the granite. The strata of the killas as well as the junction 

 between both rocks dip to the north-east at an angle of 20 de- 

 grees at the eastern side of the island ; on the northern side the 

 strata dip in the same direction at an angle of 45 degrees; on 

 the western, they underlie very rapidly to the north ; but the 

 junction of both rocks is not parallel to the strata of the killas 

 here, being nearly perpendicular. Granite veins run in every 

 direction through the killas in the neighbourhood of the junction 

 between both rocks; they start evidently from the main body of 

 the granite ; they do not present any other fact than those veins 

 hitherto described. Quartz veins occur very frequently in the 

 killas ; they both intersect the granite veins, and are intersected 

 by them. Other quartz veins, of which the mass is that kind 

 called milk quartz, are more regular ; they are from an inch 

 to an inch and a half wide, and therefore they may belong to a 

 different formation from those first mentioned. Quartz veins 

 containing mica also occui', and perhaps they may belong to 

 those veins of quartz, the description of which shall soon fol- 

 low ; but several of them may be said to belong rather to the 

 granite veins. Veins of mica, the crystals of which shoot out 

 from the wall to the interior, occur in the killas as well as in 

 the granite ; they contain a small })ortion of quartz, and run 

 generally nearly east and west. The main body of the granite 

 forming this hill is of a very light colour, containing much fel- 

 spar, but no twin crystals porphyritically dispersed through 

 the mass ; therefore it has some resemblance to the granite of 

 Carclaze, Cligga Point, and Tregonning Hill. But it partakes 

 of tiie nature of the granite at the two points first mentioned, 

 still more than by this appearance, by being intersected by 

 numberless cjuariz veins, running nearly east and west, di|)ping 

 nearly perpendicularly, containing fine crystals of oxide of tin, 

 wolfram, mica, apatite and topaz; their walls consist of a 



2 I 2 granite 



