PLUTONIC KOCKa 
551 
CHAPTER XXXI 
PLUTOOTC EOCKS. 
General Aspect of Plutonic Eocks.—Granite and its Varieties.—Decompos¬ 
ing into Spherical Masses.—Kude columnar Structure.—Graphic Granite. 
—Mutual Penetration of Crystals of Quartz and Feldspar.—Glass Cavities 
in Quartz of Granite.—Porphyritic, talcose, and syenitic Granite.—Schorl- 
rock and Eurite.—Syenite.—Connection of the Granites and Syenites with 
the Volcanic Eocks.—Analogy in Composition of Trachyte and Granite.— 
Grani;e Veins in Glen Tilt, Cape of Good Hope, and Cornwall.—Metallif¬ 
erous Veins in Strata near their Junction with Granite.—Quartz Veins. 
—Exposure of Plutonic Eocks at the Surface due to Denudation. 
The platonic rocks may be treated of next in order, as 
they are most nearly allied to the volcanic class already con¬ 
sidered. I have described, in the first chapter, these platonic 
rocks as the an stratified division of the crystalline or hypo- 
gene formations, and have stated that they differ from the 
volcanic rocks, not only by their more crystalline text are, 
bat also by the absence of taffs and breccias, which are,the 
prodacts of eraptions at the earth’s sarface, whether thrown 
np into the air or the sea. They differ also by the absence 
of pores or cellalar cavities, to which the expansion of the 
entangled gases gives rise in ordinary lava, never being sco- 
riaceoas or amygdaloidal, and never forming a porphyry 
with an uncrystalline base, nor alternating with tuffs. 
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 subterranean heat may 
extend downward 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 pla¬ 
tonic rocks, each different in texture, and sometimes even in 
composition, may originate simultaneously, the one at the 
