Ch.XXV.j 
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, 
2 A 2 
