560 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 — Mine- 

 rals 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 com- 

 position 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 unstratified division of 

 the crystalline or hypogene formations, and have stated that they differ 

 from the volcanic rocks, not only by their more crystalline texture, but 

 also by the absence of tuffs and breccias, which are the products of erup- 

 tions 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 crystal- 

 lized 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 subterranean 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, dis- 

 tinct ; so that volcanic and plutonic rocks, each different in texture, and 

 sometimes even in composition, may originate simultaneously, the one 

 at the surface, the other far beneath it. 



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

 comprehended under the name of granite, which is, then, understood to 

 embrace a large family of crystalline and compound rocks, usually found 

 underlying all other formations ; whereas we have seen that trap very 

 commonly overlies strata of different ages. Granite often preserves a 

 very uniform character throughout a wide range of territory, forming 

 hills of a peculiar rounded form, usually clad with a scanty vegetation 



