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PHYSICAL GEOGRAPHY.

CHAP. I.

depth of about ten miles. All the substances ofwhich we have any information are divided into fourclasses, distinguished by the manner in which theyhave been formed : namely,—plutonic and volcanicrocks, both of igneous origin, though producedunder ditferent circumstances ; aqueous or stratifiedrocks, entirely due to the action of water, as thename implies; and metamorphic rocks, deposited bywater, according to the opinion of many eminentgeologists, and consequently stratified, but subse-quently altered and crystallized by heat. Theaqueous and volcanic rocks are formed at the surfaceof the earth, the plutonic and metamorphic at greatdepths ; but all of them have originated simultane-ously during every geological period, and are nowin a state of slow and constant progress. The an-tagonist principles of fire and water have ever beenand still are the cause of the perpetual vicissitudesto which the crust of the earth is liable.

It has been ascertained by observation that theplutonic rocks, consisting of the granites and someof the porphyries, were formed in the deep andfiery caverns of the earth, of melted matter, whichcrystallized as it slowly cooled under enormouspressure, and was then heaved up in unstratifiedmasses by the elastic force of the internal heat evento the tops of the highest mountains, or forced ina semi-fluid state into fissures of the superin-cumbent strata, sometimes into the cracks of thepreviously formed granite: for that rock, whichconstitutes the base of so large a portion of the