Chapter V: Section I: The Classification of Rocks

In the first place, we divide rocks into _stratified_ and _unstratified_. This division is one which will in general be easily recognized, even by the most inexperienced observer; and the distinction is important, because it separates the rocks of igneous origin from those which have been produced by deposition of sediment from water.

It will be shown hereafter that a part of the unstratified rocks have been formed at or near the surface of the earth; that is, they have taken their present form by passing from a state of fusion to a solid state above or between the stratified rocks, as in the case of lava (Fig. 2, A). The other unstratified rocks have cooled so as to take the solid form below the stratified rocks, as at B. The first are called _epigene_, or _volcanic rocks_; the last, _hypogene_, or _plutonic rocks_.

The lowest portion of the second division, the stratified rocks, are termed _non-fossiliferous_, from the fact that they contain no evidence of the existence of organic beings at the time when they were deposited. Their relation to the other rocks is shown at C. It is supposed that these rocks have been subjected to great changes by heat from the igneous rocks below them. On this account Mr. Lyell proposes to call them _metamorphic rocks_. The other portions of the stratified rocks are _fossiliferous_, containing the remains of organic beings which lived at the period when the rocks were deposited. They are represented at D. The division of the last-named rocks info groups will be given hereafter.

We have then four principal classes of rocks: _Plutonic Rocks_, _Volcanic Rocks_, _Non-fossiliferous Stratified Rocks_ and _Fossiliferous Rocks_.