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CAUSES OF MAGMATIC DIFFERENTIATION.

In petrographical literature in recent years attention has repeatedly been drawn to the fact, that igneous rocks, which are closely connected geographically and in age, are also chemically related to one another, showing a certain "consanguinity"—to use Iddings'[1] very fitting expression—a relationship which makes them form a distinct "petrographical province" (Judd) when compared with igneous rocks of other parts of the world. The cause of this relationship has been sought in the supposition, that all the different rocks of the "petrographical province" come from the differentiation of one common magma, originally homogeneous.

As to the manner in which the differentiation took place, opinions are divided. We may suppose that it took place during the consolidation of the magma; in this way, a part of the minerals crystallized out, then were mechanically accumulated and finally reliquified. The differentiation of the original magma into partial magmas could take place in this way, but, as far as I can see, only on a small scale. A silicate magma during its period of crystallization is certainly too viscous to permit of any considerable diffusion. For example, in the reproduction of rocks after the method of Fouqué and Lévy, in which process a glass is first made having the desired composition, this glass may be completely devitrified (fused), while it remains so viscous that pieces of it neither change form nor adhere to one another.

Another theory, namely, that the differentiation has taken place in the magma while quite fluid, possesses greater probability and therefore more adherents. But concerning the details of the method opinions differ. While certain petrographers apply the

  1. ↑ "Origin of Igneous Rocks."Bull. Phil. Soc. of Washington, 12. 89-214. (1892).This paper contains an extensive bibliography of this subject, to which the reader is referred.

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