MINERAL RICHES OF OUR VICINITY. 69 



containing crystals of felspar, often in very irregular 

 rhombs, imbedded in a granular cement, glistening 

 with mica. The quarry opened a few years since on 

 Roborough Down, of the stone formerly used in 

 building there, called Roborough Down stone, (of 

 which, indeed, many of the neighbouring churches are 

 constructed,) is a porphyry, of a very curious kind, 

 the mineralogical composition of which has not yet 

 been determined ; and several other rocks, under this 

 name, may be traced in our precincts, as at Morwhell 

 Down, Sec. But one of these, that at Mount Edge- 

 cumbe, has yet been examined by the mineralogist ; 

 and as that is not quarried, nothing of course can be 

 found there but what has been weather-beaten for ages. 

 In fact they are so little quarried (except at Grenofen, 

 for mending the road) that the best places to seek por- 

 phyritic minerals will usually be the mining heaps, 

 where that rock happens to come in contact with the 

 lode. These heaps will be further alluded to in treat- 

 ing of the slate. But there can be little danger of the 

 collector's losing his labour entirely, either at Grenofen, 

 Walkhampton, or any other recently opened porphyri- 

 tic stratum. To give a list of the numerous minerals 

 which might be expected there, would be superfluous ; 

 particularly as many of them will occur in the granite 

 and trapp, about to be described. 



4. The Granite contains crystals of quartz, mica, 

 and felspar, as its essential composition ; all of which 

 are found more or less perfect and detached, in the 

 occasional natural cavities of the stone. Beryl, zircon, 

 topaz, and tourmaline have also been found in the 

 granite of Dartmoor : the latter is most common in the 

 red granite, and therefore likely to be met with at 

 Trowlsworthy, and Nether Shaugh ; in neither of which 

 places it seems yet to have been noticed. Tin mines 

 are chiefly found in granite ; and tin ore, though having 

 but few varieties, produces, when crystallized, some 

 very fine specimens. Wood tin, and shot tin, as they 

 are called, are found in the stream works. Wolfram 

 also is found in tin mines ; and red oxide of iron, as 



VOL. III. 1634. K 



