PART I. CHAPTER IX. 



127 



Mineral Structure of Granite in Veins. 



nular 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. 117. 



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



Fig. 117. is a sketch of a group of granite veins in Cornwall, 

 given by Messrs. Von Oeynhausen and Von 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 six- 

 Fig. 118. ,,////////////!& teen to twenty feet, 



but some are much 

 higher. 



In the Valorsine, 

 a valley not far 

 from Mont Blanc, 

 in Switzerland, an 

 ordinary granite, 

 consisting of fel- 



reins of granite talcose gneiss. (L. A. Necker.) sparj quartZ) and 



mica, sends forth veins into a talcose gneiss, (or stratified proto- 

 gine,) and in some places lateral ramifications are thrown off 

 from the principal veins at right angles (see Fig. 118.) the veins, 

 especially the minuter ones, being finer grained than the granite 

 in mass. 



It is here remarked, that the schist and granite, as they approach, 

 seem to exercise a reciprocal influence on each other, for both 



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

 t Phil. Mag. and Annals, No. 27. New Series, March, 1829. 



