When we examine in the field the igneous rocks in their relations to other rock-masses, we frequently find cases where it is exceedingly difficult to account for the presence of the igneous mass, except upon the assumption that the magma made way for itself by fusing and incorporating the rocks which must formerly have occupied its present position, the surrounding rocks showing no evidence of being merely pushed aside by the ascending magma. In certain instances, such a melting and incorporating of opposing rocks would seem to be clear, as when a sheet of magma has made its way into a series of strata, parallel with the bedding planes, without increasing the thickness of the series. This incorporating of freshly fused material with the intruding magma is called assimilation, but, save on a very small scale, its reality is a subject of much dispute, and some of the highest authorities altogether reject it. Nevertheless, many observed facts strongly favour this assimilation theory, which has a most important bearing upon some of the fundamental problems of geology. , According to this view, the ascending magma is at an extremely high temperature and very fluid, and it forces its way upward partly through crevices and fissures, partly by detaching joint-blocks, which sink into the molten mass and are dissolved by it, thus greatly modifying the chemical constitution of the magma.

Subsequently, the magma becomes differentiated, so that the different varieties of rock separate from one another in the manner already described. Perhaps, as Daly has suggested, there is a universal subcrustal magma, of basic composition, which, owing to the pressure of the overlying crust, is only potentially fluid, liquefying along lines of a partial release of pressure. The overlying solid rocks are, on the average, more acid than the subcrustal magma and thus, by assimilation followed by differentiation, the many varieties of igneous rock are formed.