Granite veins traversing gneiss at Cape Wrath, in Scotland. (MacCulloch.)
The accompanying sketches will explain the manner in which granite veins often ramify and cut each other ([figs. 494.] and [495.]). They represent the manner in which the gneiss at Cape Wrath, in Sutherlandshire, is intersected by veins. Their light colour, strongly contrasted with that of the hornblende-schist, here associated with the gneiss, renders them very conspicuous.
Granite very generally assumes a finer grain, and undergoes a change in mineral composition, in the veins which it sends into contiguous rocks. 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 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.[444-C]
[Fig. 496.] is a sketch of a group of granite veins in Cornwall, given by Messrs. Von Oeynhausen and Von Dechen.[445-A] 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.
Fig. 496.
Granite veins passing through hornblende slate, Carnsilver Cove, Cornwall.
In the Valorsine, a valley not far from Mont Blanc in Switzerland, an ordinary granite, consisting of felspar, quartz, and mica, sends forth veins into a talcose gneiss (or stratified protogine), and in some places lateral ramifications are thrown off from the principal veins at right angles (see [fig. 497.]), the veins, especially the minute ones, being finer grained than the granite in mass.