THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 569 



of the liquid by precipitating their heat equivalent of the phases 

 with which the magma is saturated (basic minerals) . The inclu- 

 sions, it should be noted, must be of a composition toward which 

 the liquid could pass spontaneously by fractional crystallization. 

 Thus saturated basaltic magma can dissolve granitic inclusions by 

 precipitating basic minerals and the granitic material passing into 

 solution then becomes a contribution to the normal granitic differ- 

 entiate that may form by fractional crystallization if the conditions 

 are appropriate. 



The behavior of inclusions of sedimentary origin is more compli- 

 cated since sedimentary material does not belong in the reaction 

 series. A consideration of the extent and nature of the variation of 

 composition possible in the crystalline phases formed from a magma 

 shows that the incorporation of considerable amounts of sedimentary 

 material would ordinarily bring about merely an adjustment in the 

 composition and relative proportions of existing phases. As a result 

 of the non-appearance of new phases, the general course of fractional 

 crystallization is unaffected. In general, the adjustment noted 

 takes place through precipitation of the phases with which the 

 magma is saturated. As an example it may be stated that the 

 addition of highly aluminous sediments to basic magma should bring 

 about the formation of anorthite and enstatite molecules at the 

 expense of diopside molecules and should therefore cause the precipi- 

 tation of crystals rich in anorthite and enstatite. Such action may 

 have been important in the formation of many norites. The foreign 

 material becomes a part of the general mass as a result of reaction 

 and precipitation rather than by simple solution. 



The Cortlandt series of New York, with its inclusions, affords an 

 illustration of the behavior of aluminous sediments in basic magma. 

 Such sediments may be regarded as consisting in part of material 

 corresponding in composition with igneous material late in the reac- 

 tion series, together with a certain excess, which is highly aluminous. 

 The former may become a part of the liquid by the method of reac- 

 tive solution already described. There results the piling-up of the 

 highly aluminous excess in the inclusions, with formation of such 

 minerals as sillimanite. Moreover, as a consequence of what may 

 be somewhat loosely called the instability of sillimanite in contact 



