Ch.XXV.] GRANITE VEINS. 355 



representations of the granite veins in Scotland, in which 

 the contrast of colour between the vein and some of the dark 

 varieties of hornblende-schist associated with the gneiss renders 

 the phenomena more conspicuous. 



The following sketch of a group of granite veins in Cornwall 

 is given by Messieurs Von Oeynhausen and Von Dechen*. 



No. 87. 



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



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. 



The vein-granite of Cornwall very generally assumes a finer 

 grain, and frequently undergoes a change in mineral com- 

 position, as is very commonly observed in other countries- 

 Thus, according to Professor Sedgwick, the main body of the 

 Cornish granite is an aggregate of mica, quartz, and felspar ; but 

 the veins are sometimes without mica, being a granular aggre- 

 gate 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 f. 



Changes are sometimes caused in the intersected strata very 



* Phil, Mag. and Annals, No. 27, new Series, March, 1829. 

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



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