The unstratified or massive rocks have risen in a molten state from below toward the surface, though by no means always reaching it, and have forced, or perhaps have sometimes melted, their way through or between the stratified rocks. One of the most important points to determine with regard to a massive rock is its relation to the strata in which it occurs; for the earth's chronology is given by the stratified rocks. Considered only with reference to itself, an igneous mass gives no trustworthy evidence as to the time when it was formed. The term eruptive is frequently employed in the same sense as unstratified, because of the belief that most igneous masses have been connected with volcanoes; but as such a belief may not be well founded, it is better to use a non-committal term.
As in most departments of geology, there are unfortunately considerable differences in the meaning attached by various writers to the terms used in the description of the igneous, or massive, rocks. Since it is highly desirable that greater uniformity and exactness of nomenclature should be attained, the usage proposed by Professor Daly will be followed here, though his classification is more elaborate than is required in an elementary work.
We shall first take up the volcanic rocks, because modern volcanoes give us the key by which we may readily interpret them.