THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 557 



cyanite. That such minerals together with corundum could form 

 from basic magnesian rocks by simple differentiation is very doubt- 

 ful.^ 



The progressively selective nature of the reaction between 

 magma and inclusions that is shown by these Cortlandt examples 

 and the theoretical considerations discussed in connection therewith 

 demonstrate that a former statement of mine that "the formation 

 of an obviously hybrid rock should, apparently, be the normal result 

 of assimilation"^ is erroneous and that Daly's objections thereto are 

 justified.^ Daly 's objections are based on considerations other than 

 those raised above and refer rather to the changes that may occur in 

 a superheated magma during the slow diffusion into it of xenolithic 

 material. We may recall here that the reactions herein described 

 are those that are to be expected between saturated magma and 

 schist inclusions. The fact that the reactions are selective in a way 

 that matches the expectation may presumably be regarded as evi- 

 dence that the magma was saturated. 



Recognition of the probable saturated condition of the magma 

 is of importance because it shows that there could not have been 

 formed, at any time, a liquid whose composition was simply the 

 suxn of that of the original magma and the inclusions. Even that 

 portion of the foreign matter which becomes a part of the liquid does 

 so only by precipitating phases with which the magma is saturated 

 and must itself be of a composition toward which the liquid may go 

 spontaneously by fractional crystallization. So in the case of the 

 Cortlandt series various diorites, some syenite and a considerable 

 amount of granite (first recognized by Berkey as a part of the series) ^ 

 were formed by differentiation as they would have been under the 

 same conditions without reaction with slate material, though pre- 

 sumably the amount of "acid" differentiates was augmented by its 

 addition. 



Another suggestion concerning the incorporation of slaty rocks 

 in basic magmas may be made at this point. In the neighboring 



'■ See Rankin and Merwin, "The Ternary System MgO-AljOs-SiOa," Amer. Jour. 

 Sci., Vol. XLV (1918), p. 325. 



^ N. L. Bowen, Jour. Geol., Supplement to Vol. XXIII (1915), p. 85. 



3 R. A. Daly, Jour. Geol., Vol. XXIII (1918), p. 126. 



4 C. P. Berkey, Science, Vol. XXVIII (1908), p. 575. 



