88 JOURNAL OF GEOLOGY—SUPPLEMENT 



But there are doubtless some cases in which the syntectic 

 magma formed by assimilation crystallizes only toward its borders 

 as a hybrid rock, whereas, in adjacent portions of the magma the 

 syntectic crystallizes sufficiently slowly to permit of differentiation 

 and, possibly, the masking of hybridism. It is probable that in 

 some cases a magma has locally completely incorporated an amount 

 of foreign material equal to a small percentage of its own mass, and 

 the question arises as to the effect on the course of differentiation. 



It is clear that if the foreign material is igneous, the syntectic 

 magma resulting will crystallize according to the general scheme 

 oiithned for a normal magma. No unusual result is possible. 

 The only effect will be such that if the material added is, say, 

 granitic, the granitic differentiate will be correspondingly increased, 

 provided, of course, cooling is sufficiently slow to permit the 

 attainment of the granitic stage. 



If the foreign material is of sedimentary origin the result will in 

 general be not very different. Even when the sedimentary rock 

 has a composition considerably removed from that of any possible 

 igneous rock nothing is added to the magma that it does not already 

 contain. But since the added material contains the various oxides 

 in proportions very different from those of the magma, the result 

 will be a rearrangement of equilibrium in the liquid. Certain 

 combinations already present will be increased in amount, others 

 diminished. When only a moderate amount of material has been 

 added, no special result will ensue. Crystallization will follow the 

 normal course described, but at certain stages there will be increased 

 or decreased precipitation of some of the normal minerals accord- 

 ing to the nature of the material added. The result will not be 

 distinguishable from any normal suite of igneous rocks. 



In an extreme case, however, a great increase may result in 

 certain combinations which normally do not occur in the magma 

 in sufficient concentration to be represented among the minerals 

 precipitated and are therefore not generally found as crystals in 

 igneous rocks. In such a case one or more of these combinations 

 may exceed its solubility at some stage in the history of the magma 

 and, being precipitated, will be added to the normal minerals. Such 

 combinations are likely to be the same as those which form in the 



