CHAPTER XXXIII.

PLUTONIC ROCKS--GRANITE.

General aspect of granite--Decomposing into spherical masses--Rude columnar structure--Analogy and difference of volcanic and plutonic formations--Minerals in granite, and their arrangement--Graphic and porphyritic granite--Mutual penetration of crystals of quartz and felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly granites--Eurite--Passage of granite into trap--Examples near Christiania and in Aberdeenshire--Analogy in composition of trachyte and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and other countries--Different composition of veins from main body of granite--Metalliferous veins in strata near their junction with granite--Apparent isolation of nodules of granite--Quartz veins--Whether plutonic rocks are ever overlying--Their exposure at the surface due to denudation 436