1813.] Minerulogkal Observations on Cornwall. 347 



shelving down towards the sea. This rock is thick slaty, and 

 has very much the appearance of clay slate, but is considerably 

 harder than any clay slate which I ever saw. It is dark blue, 

 gives a white streak, and has little or no lustre. It will be con- 

 sidered, I presume, as a variety of clay slate ; at least I do not 

 know any other rock to which it can be so well referred. 



The pier of Penzance is situated near the south end of the 

 town. It is built of granite, and is just finished. Beds of a 

 granitic rock are seen rising out of the sea just behind this pier. 

 It is a remarkably hard rock, of a brownish grey colour, and 

 consists chiefly of felspar, in which crystals of transparent quartz 

 are imbedded, which give it the appearance of a porphyry. 

 Mica is very sparingly distributed through this rock; you may 

 examine different specimens before you perceive any. Over this 

 granitic rock lies a bed of clay slate. It is thick slaty, like the 

 bed at the north end of the town ; but its texture is coarser, it is 

 not so hard, and has a purplish colour. 



These two clay slate rocks were the only ones of the kind that 

 I observed in the whole peninsula, upon the east side of which 

 Penzance is situated. They have none of the peculiar characters 

 of primitive clay slate, but approach much more nearly to those 

 of transition slate. Hence 1 am disposed to consider them as 

 transition.. They lie immediately over the granite rock, which, 

 as far as I was able to determine, seems clearly to constitute the 

 fundamental rock of this peninsula. Between Penzance and the 

 Land's End, a distance of ten miles, no other stone but granite 

 was to be seen. Granite blocks were thick scattered over the 

 fields, they constituted the material of which all the stone hedges 

 are constructed, and I met here and there with rocks of gianite 

 quarried, in order to procure materials for mending ihe roads. 

 This granite is porphyritic, the felspar is brown, the quartz 

 white, and the mica black. It contains numerous large crystals 

 of white felspar, three or four inches long, and often two or 

 three inches broad. These crystals give the stone a peculiar and 

 rather beautiful appearance. It is common to observe black 

 irregular shaped patches in this granite, which when seen at a 

 distance resemble fragments of other rocks; but when examined 

 more closely, they are found to consist of scales of black mica 

 thick set in a base of quartz. I thought, at the time, (hat I per- 

 ceived likewise some hornblende in this granite, and broke off 

 some specimens on that account ; but upon examining these 

 miens more tan-fully at Penzance, I found that the black 

 matter was not hornblende, but mica. This kind of granite 

 continues all the way to the Land's End. The rocks composing 

 that promontory consul of it. They are perpendicular cliffs 

 above 200 feet high ; and the aspect which they have acquired, 

 from long exposure to the weather, puts oiic in mind of the face 



