We now come to a series of rocks which no one has ever observed in the course of formation, because they were solidified at greater or less depths underground. When such masses are exposed to view, it is not because they have been brought to the surface, but because the surface has been eroded down to them. Though these unstratified masses cannot be observed in the process of formation, as may the lavas and pyroclastic rocks, vet the nature of the rocks themselves, and their relations to the volcanic and stratified rocks, enable us to explain them satisfactorily. In whatever shape they occur, these masses are intrusive, and have forced or melted their way upward, filling fissures and cavities, or have thrust themselves between strata, following the path of least resistance. Intrusions are younger, it may be vastly so, than the strata which they penetrate and lie over or beneath; their geological date may be determined by a process of elimination, finding the newest strata which they have traversed and the oldest which they have not reached.
A primary division of the plutonic masses is into (1) injected and (2) subjacent bodies. "An injected body is one which is entirely enclosed within the invaded formations, except along the relatively narrow openings to the chamber where the latter has been in communication with the feeding reservoir." (Daly.) Subjacent bodies, on the other hand, have no floor upon which the intrusive mass rests, the communication with the earth's interior being by great openings which enlarge downward indefinitely within the limits of observation.
Both injected and subjacent bodies may be either simple, i.e. composed of material intruded at one period, multiple, i.e. composed of material of the same kind intruded at more than one period, or composite, i.e. made up of material derived from different kinds of magma intruded at more than one period of time.
These are of manifold variety of shapes and sizes and differ in their relations to the enclosing, or country rock, and different terms are accordingly used to describe them.