22 GRANITIC ROCKS OF CORNWALL. t Ch - IL 



granite : they are numerous, and, when they meet, they do 

 not traverse each other, but unite.* 



Mr. Majendie, in noticing the veins and concretions of 

 fine-grained granite and shorl-rock, in the porphyritic granite 

 of the Logan Rock, remarks, that these, which on a slight 

 view might be taken for fragments, are often penetrated by 

 large crystals of felspar proceeding from the granite mass, f 



Some examples of these granite-veins, on the same coast, 

 have been described by Oeynhausen and Dechen. The 

 granite of Tol-Pedn-Penwith, they observe, is of the 

 common kind, with porphyritic twin crystals of felspar 

 with shorl and pinite. Near the village of Sawah the fine- 

 grained granite is like that of veins, consisting of quartz and 

 red felspar, with a little mica, but a larger quantity of shorl : 

 its position, however, is not that of a vein. The mass of the 

 fine-grained rock continues of a fine texture to the distance 

 of twenty feet from its junction with the large-grained ; but, 

 farther off, the constituent parts become larger and larger, 

 and porphyritic crystals of felspar appear here and there ; so 

 that this rock gradually passes into the large-grained variety, 

 both sorts of granite only differing from each other by their 

 texture, and different state of crystallisation. Several granite- 

 veins, exactly of the same composition as the fine-grained just 

 described, abound in the cliff: one of these, in a little cove 

 near the signal-station, is heaved nearly two feet by a quartz 

 vein, as is also a shorl vein; but the latter traverses the 

 granite-vein, without producing any heave. This granite- 

 vein continues for a considerable distance 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.:}: 



When these granite-veins are of a large size they are 

 termed elvan-courses : indeed, this is the only distinction be- 



Geol. Trans. Cornwall, vol. ii. p. 54. f Idem, vol. i. p. 29. 



Phil. Mag. and Annals, vol. v. p. 224. 



