246 0« the Junction of the Grajiite and Killas in CornwaU. 



Penwith consists only of the common sovtof porphyritic granite; 

 in the eastern cove occurs a vein of fine-grained granite. In 

 a little cove on the west of the signal placed on the promontory 

 is a quartz vein one foot wide, running north-west and south- 

 east, and dipping to south-west at an angle of 66 degrees ; a 

 little gossan joins it, and it seems evident that this quartz vein 

 belongs to -johat the Cornish miner calls a lode. This quartz in- 

 tersects a granite vein (fig. 30.) of the breadth of three or four 

 inches, and heaves it neax'ly two feet : the same granite vein is 

 intersected by a schorl vein without being heaved ; and this 

 schorl vein also is intersected and heaved by the quartz vein. 

 The granite vein continues to a very considerable extent into 

 the sea; it is here divided into two branches; the large twin 

 crystals of felspar they meet with, are intersected by them, and 

 heaved about half an inch. This granite vein contains only 

 a small portion of mica, but in some places much schorl, and 

 takes the appearance of a schorl vein. The schorl vein has had 

 influence on the granite in its walls, where the felspar is found 

 to be changed into China clay. 



The killas overlies the granite; and the granite must have 

 formed a basis on which the killas could be deposited ; there- 

 fore granite may be considered as the oldest rock in this coun- 

 try. Other masses of granite, Irom which the granite veins in 

 the killas and the granite itself strike offj appear to have been 

 introduced at a more recent period in the granite first men- 

 tioned, and evidently after the formation of the killas in which 

 the granite veins are found. Masses of granite of this kind may 

 occur very frequently; and perhaps the granite of St. Michael's 

 Mount, of Cligga Point, of Carclaze, and of Tregonning Hill, 

 may belong to that granite afterwards introduced into the solid 

 of the rock : but to prove it would be very difficult ; because the 

 position of these granite masses is not so well laid open to the 

 view as at Tol Pedn Penwith. It will appear evident that 

 at the junction of both kinds of granite only is it possible to 

 distinguish them ; the interior of both being formed in such 

 a manner that they cannot be distinguished. The granite 

 veins, the schorl veins, and the tin lodes in the granite, appear 

 to belong to the oldest formation of veins which has taken 

 place in these countries. The copper lodes, and those tin 

 lodes which occur in the killas, and commonly contain some 

 copper ore, belong to a later formation, and we should think 

 both belong to the same formation. 



The killas is, at its contact with the granite, rather horn- 

 blende-slate and greenstone than clay-slate; in the Lizard 

 district we have seen the greenstone intimately mixed with 

 crranite, both occurring even in the same vein. The transition 



irom 



