Ch. xxxin.] 



GRANITE IN VEINS. 



713 



felspar ; but the veins are sometimes without mica, being a granular 

 aggregate of quartz and felspar. In other varieties quartz prevails to 

 the almost entire exclusion both of felspar and mica ; in others, the 

 mica and quartz both disappear, and the vein is simply composed of 

 white granular felspar.* 



Fig. 744 is a sketch of a group of granite veins in Cornwall, given 

 by Messrs. Von Oeynhausen and Yon Dechen.f The main body of 

 the granite here is of a porphyritic appearance, with large crystals of 

 felspar ; but in the veins it is fine-grained, and without these large 

 crystals. The general height of the veins is from 16 to 20 feet, but 



some are much higher. 



Ffe. 744. 



Granite veins passing through hornblende slate, Carnsilver Cove, Cornwall. 



In the Valorsine, a valley not far from Mont Blanc in Savoy, an 

 ordinary granite, consisting of felspar, quartz, and mica, sends forth 

 veins into a talcose gneiss (or stratified protogine), and in some 



Fig. 745. 



Veins of granite in talcose gneiss. (L. A. Neeker.) 



places lateral ramifications are thrown off from the principal veins at 

 right angles (see fig. 745), the veins, especially the minute ones, being 

 finer grained than the granite in mass. 



* On Geol. of Cornwall, Camb. Trans., vol. i. p. 124. 



f Phil. Mag. and Annals, No. 27, new series, March, 1829. 



