COMPOUND MINERALS,
OR
ROCKS.
249. There exist considerable masses of minerals in a state of combination, or aggregation with each other. These constitute the rocks and soil of which the globe of the earth is composed; and the study of them is called GEOLOGY. The opinions of learned men relative to their structure, and original formation, have produced various systems denominated theories of the earth; but, when we consider that the greatest depth beneath the surface to which the art and industry of man have been able to penetrate, does not exceed 1/35000 part of the earth’s diameter, we must confess that this is very insufficient to allow of any correct opinion being thereby formed concerning the structure of the whole.
Modern geologists, for the more convenient arrangement of the compound minerals, have divided them into four classes, which they denominate primitive rocks, secondary rocks, alluvial depositions, and volcanic rocks.
I. PRIMITIVE ROCKS.
250. These are so called from their being considered by geologists, to belong to the first formed parts of the globe.
Rocks of this description are of a nature extremely hard. They contain no vestiges whatever of animal or organic remains; and the substances of which they are composed are crystallized. They rise through other rocks at various elevations, in every quarter of the globe; and never either alternate with, or rest upon rocks that enclose organic remains, though they are themselves frequently covered by such.
The following are the principal kinds.