The igneous rocks have a deep-seated origin and have either forced their way to the surface, or have cooled and solidified at varying depths beneath it. Though rocks of this class, there is every reason to believe, were the first to be formed, they have been made all through the recorded history of the earth, and, as volcanoes show, are forming now. They are thus the primary rocks and all the others have been derived, either directly or indirectly, from them. The products of the chemical decomposition or mechanical abrasion of the igneous rocks have furnished the materials out of which the sedimentary rocks were formed, at least in the first instance.

The igneous rocks are massive, as distinguished from stratified, and though sometimes presenting a deceptive appearance of stratification, may always, with a little care, be readily distinguished from the truly stratified rocks. The term massive is, indeed, frequently used for these rocks in the same sense as igneous, and eruptive rocks is another term meaning the same thing, though eruptive is also employed in a more restricted sense. Still another term which should be defined is magma, by which is meant a continuous molten mass before solidification.

Characteristic differences appear between those igneous masses which have solidified deep within the earth and have been brought to light only by the denudation and removal of the overlying rock-masses, and those which have cooled at or near the surface of the ground. The former are called plutonic (abyssal, or intrusive) and the latter volcanic (or extrusive). Between the two may be found every gradation, and the term hypabyssal is sometimes employed for rocks which are transitional between the typical plutonic and the typical volcanic kinds. These terms, plutonic, hypabyssal, and volcanic, are used to describe the character of rock-masses, not as terms of classification.