702 PLUTONIC ROCKS. [Ch. XXXIII. 



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. 



The plutonic rocks may be treated of next in order, as they are 

 most nearly allied to the volcanic class already considered. I have 

 described, in the first chapter, these plutonic rocks as the ttnstratified 

 division of the crystalline or hypogene formations, and have stated 

 that they differ from the volcanic rocks, not only by their more crys- 

 talline texture, but also by the absence of tuffs and breccias, which 

 are the products of eruptions at the earth's surface, or beneath seas 

 of inconsiderable depth. They differ also by the absence of pores 

 or cellular cavities, to which the expansion of the entangled gases 

 gives rise in ordinary lava. From these and other peculiarities it has 

 been inferred, that the granites have been formed at considerable 

 depths in the earth, and have cooled and crystallized slowly under 

 great pressure, where the contained gases could not expand. The 

 volcanic rocks, on the contrary, although they also have risen up 

 from below, have cooled from a melted state more rapidly upon or 

 near the surface. From this hypothesis of the great depth at which 

 the granites originated, has been derived the name of " Plutonic 

 rocks." The beginner will easily conceive that the influence of sub- 

 terranean heat may extend downwards from the crater of every active 

 volcano to a great depth below, perhaps several miles or leagues, and 

 the effects which are produced deep in the bowels of the earth may, 

 or rather must, be distinct ; so that volcanic and plutonic rocks, each 

 different in texture, and sometimes even in composition, may origi- 

 nate simultaneously, the one at the surface, the other far beneath it. 



By some writers, all the rocks now under consideration have been 



