rock, with the waters of the present seas and the present atmosphere
around it, though these were probably in considerably different
conditions, both as to temperature and their constituent materials,
from what they now are. We are thus to presume that that crystalline
texture of rock which we see exemplified in granite is the condition
into which the great bulk of the solids of our earth were
agglomerated directly from the nebulous or vaporiform state. It is a
condition eminently of combination, for such rock is invariably
composed of two or more of four substances--silica, mica, quartz, and
hornblende--which associate in it in the form of grains or crystals,
and which are themselves each composed of a group of the simple or
elementary substances.

Judging from the results and from still remaining conditions, we must
suppose that the heat retained in the interior of the globe was more
intense, or had greater freedom to act, in some places than in
others. These became the scenes of volcanic operations, and in time
marked their situations by the extrusion of traps and basalts from
below--namely, rocks composed of the crystalline matter fused by
intense heat, and developed on the surface in various conditions,
according to the particular circumstances under which it was sent up;
some, for example, being thrown up under water, and some in the open
air, which conditions are found to have made considerable difference
in its texture and appearance. The great stores of subterranean heat
also served an important purpose in the formation of the aqueous
rocks. These rocks might, according to Sir John Herschel, become
subject to heat in the following manner:- While the surface of a
particular mass of rock forms the bed of the sea, the heat is kept at
a certain distance from that surface by the contact of the water;
philosophically speaking, it radiates away the heat into the sea, and