Origin of Igneous Rocks

Abstract

It has been shown that the composition of igneous rocks as a whole varies widely within certain definite limits (p. 46). This diversity may be shown in a number of related but separate intrusions or extrusions; or it may appear within a single rock mass. Reasons have been given for the assumption that there are only a few, perhaps only two, primary magmas (p. 53). Two problems are thus involved: namely, that of the derivation of the present diversity of igneous rock types from simple initial magmas; and that of the ultimate origin of the primary magmas themselves. With regard to the latter, only speculation can, as yet, be offered, and it is not proposed to deal with this question here; but the problem of the immediate origin of igneous rocks has already been more or less successfully tackled. The present variations of igneous rocks may be ascribed to two causes: differentiation, and assimilation (or syntexis). Differentiation may be defined as the process whereby a magma, originally homogeneous, splits up into contrasted parts, which may form separate bodies of rock, or may remain within the boundaries of a single unitary mass.