As in all questions which deal with the subterranean agencies, the exact manner in which molten magmas make their way up 2D through the overlying rocks is veiled in obscurity; in fact, it is the unsolved problem of the ascensive force of lava in another shape. The great variety of forms assumed by the intrusive bodies is due to the complex interaction of two main groups of factors - the ascensive force of the molten magma, however that may be generated, and the resistance to be overcome. With these are frequently associated factors of a third series, the orogenic compression of the rocks, which may squeeze a purely passive magma into the cavities and fissures made by compression. The description given in the preceding section of the various plutonic bodies left out of account the fact that the different kinds are connected by all sorts of transitions. Laccoliths grade into sills, on the one hand, and into chonoliths, on the other, and the same continuous body may be a dyke in part of its course, a sill in another, and so on. The character of the magma itself is also of importance in determining the result, whether the molten mass is thoroughly fluid or merely pasty, and how great the quantity of the imprisoned gases and vapours.

In the complicated play of these different factors it is often extremely difficult to distinguish effect from cause, and it is this which gives rise to such radical divergences of opinion in interpreting the phenomena.

Fig. 221. - Mato Tepee, South Dakota, the core of a laccolith composed of columnar phonolite.

Igneous intrusions are most abundant in regions of disturbed rocks, and we find great areas of nearly horizontal strata, such as the Great Plains, in which intrusions are not known to occur. On the other hand, folded, even intensely compressed, strata may have no igneous rocks associated with them. The Appalachian Mountains, for example, are singularly free from intrusions. These associations have been differently interpreted. According to one view, the very general coincidence of extensive intrusions and orogenic compression implies that the magma is for the most part passive, and is squeezed by the compressing force into the actual or potential cavities which are generated by the compression. On the other hand, there is a growing tendency among many geologists to regard the deep-seated magmas as active and energetic agents of dislocation and to find in them the origin of the compressive force itself. We have met with this tendency already in the discussion of earthquakes (p. 52), dislocations (p. 367), etc., and it must be reckoned with in all attempts to solve the great problem of subterranean activities.

As so often happens, it will probably be found that the truth lies between the extreme views.

In the chapter on the igneous rocks (p. 291) we learned that very different opinions were held concerning the proper answer to the question whether the magmas make their way entirely by mechanical means, taking advantage of fissures, cavities, and lines of weakness, and forcing the country rock aside, or whether they may make room for themselves by dissolving, fusing, and incorporating more or less of the rocks which formerly occupied the position now held by the plutonic bodies. So far as the injected masses are concerned, it is seldom necessary to assume that they have done more than lift or push aside the enclosing rock, but the case is very different with the subjacent masses. Frequently the contact between the country rock and a stock or batholith shows no evidence that the former has been compressed or crowded to make room for the intrusion, and it seems impossible to account for the presence of the plutonic mass except on the assumption that space has been gained by fusing more or less of the enclosing country rock. It is not necessary to suppose that such fusion takes place only on the periphery of the intruding magma; on the contrary it seems more likely that the magma dislodges the joint-blocks which then sink in the highly heated mass and are gradually dissolved.

It must be admitted that this hypothesis has not been established. Some of the highest authorities maintain that it is definitely disproved by the microscopic and chemical examination of the batholithic rocks, which are not affected by the character of the country rock through which they break. We have here a conflict of evidence which it remains for future studies to harmonize. The energy of intrusion is eloquently displayed along the margins of many batholiths, where the country rock is shattered and great blocks are torn off and embedded in the plutonic mass. Such blocks are called inclusions or xenoliths, and, on a small scale, they occur in other plutonic bodies, such as sills and laccoliths. The existence of these blocks in their undissolved state has been made an argument against the assimilation hypothesis, but it must be remembered that the intact xenoliths are products of the dying energy of intrusion, when the magma was already so stiff that the blocks were no longer able to sink in it.

Fig. 222. - Inclusions (xenoliths) of schist in granite. (U. S. G. S).