THE MINING DISTRICTS OF CORNWALL. 327 



having the form of plagioclase, intermixed with a little altered 

 augite and a small quantity of more or less peroxidized and hydrated 

 magnetite, through which are disseminated a few felspathic crystals 

 of larger dimensions. The majority of these crystals are evidently 

 plagioclase • but some of them do not exhibit, with polarized light, 

 the striations characteristic of triclinic felspars. Many of the latter 

 are traversed by numerous fissures, filled with carbonate of calcium 

 &c, and, when decalcified, exhibit the appearance of disjointed irre- 

 gular fragments. From the small amount of potash in the rock, it 

 is probable that some of these may be plagioclase so altered as not 

 to afford the usual distinctive striae. Others appear to be unaltered, 

 and are probably orthoclase. Similar crystals of orthoclase occur in 

 the lavas of Ischia and of Vesuvius *. The vacuities, the filling of 

 which consists principally of calcite, are not unfrequently lined with 

 chlorite. This appears to have been formed previously to the intro- 

 duction of the former mineral ; and a flocculent greenish mineral is 

 also sparingly disseminated throughout the crystalline base. This 

 rock cannot, I think, be regarded otherwise than as being an altered 

 doleritic lava. 



A portion of the lime and magnesia of the original rock has evi- 

 dently been converted into carbonates ; and the proportion of alumina 

 is somewhat higher than is usual in rocks of this class. This might, 

 however, be anticipated when the large proportion of felspar to 

 augite, and the amount of alteration which the rock has undergone, 

 are considered. 



At a distance of about half a mile south of the Trelill quarry is 

 another, now abandoned, but which was, until recently, worked 

 for road-material. This has been opened, through slate, to a thick 

 bed of rock, somewhat similar in appearance to the foregoing ; 

 but it has a schistose structure, is softer, and also much less 

 cavernous. 



The results of a duplicate analysis of this rock, calculated in the 

 same way as before, may be thus rendered (sp. gr.=2 # 78) : — 



Proportion of carbonates to rock — 



Carbonates 12-01 



Eock 87-99 



100-00 

 The carbonates are composed of 



Carbonate of calcium 98*36 



„ magnesium 1-64 



100-00 



The percentage composition of this rock, deduction made of the 

 carbonates, is as follows : — 



* Dana, 'System of Mineralogy,' p. 360. 



