Page:A book of the west; being an introduction to Devon and Cornwall.djvu/204

156 Some granite is very coarse, full of what are locally called "horse-teeth," crystals of felspar, other is finely grained. Some is black with schorl, some, as that of Mill Tor, white as statuary marble. Granite was not well stirred before it was protruded to the surface. The constituents of granite are quartz, felspar, and mica; the latter sometimes white, at others usually black and glistering. The felspar may be recognised as being a dead white. The black shining matter found near where are veins of tin, is schorl.

It is the opinion of modern geologists that the granite never saw daylight till cold and consolidated, and that granite when in fusion and erupted to the surface resolves itself into trap. The pressure of superincumbent beds prevented perfect fusion. In its altered condition when perfectly fused it may be seen in Whit Tor, near Mary Tavy.

But, it may be asked, what has become of the beds that overlay the granite? They have been washed away. In Exmoor we do not meet with the granite. It had heaved the slates, but not sufficiently to so dislocate them as to enable the rains and floods to carry them away and reveal the granite below. If Dartmoor granite could but have retained its covering matter, the region would have been indeed mountainous. In Shavercombe, a lateral valley of the upper Plym, may be seen traces of the original coverlet of slate, much altered by heat.

The granite looks as though stratified, but this is deceptive. It is so unequally mixed that some flakes or layers are harder and more resistant to