THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS $39 



of such higher members but can only react with them to convert 

 them into that member of the reaction series with which it is satu- 

 rated, often by passing through other members of the series as inter- 

 mediate steps. The material used to effect these changes cannot 

 be regarded as simply subtracted from the liquid for the liquid is not 

 free to become impoverished in any random substance. In general 

 impoverishment in any substance will cause the liquid to pass within 

 a region of saturation and induce the precipitation of some of the 

 phases with which the liquid is saturated. 



Let us take, for example, a magma saturated with biotite, say, a 

 granitic magma. This magma is effectively supersaturated with 

 olivine,, pyroxene and amphibole and cannot dissolve them in spite 

 of the marked contrast of composition, which is often supposed to be 

 an aid to the solution of inclusions. But the magma can and will 

 react with these minerals and convert them into biotite, usually by 

 steps. The subtraction of material necessary to produce biotite 

 will cause the precipitation of the minerals with which the magma 

 is saturated until either the liquid or the inclusions are used up or the 

 reaction is brought to an end on account of mechanical obstruction. 



Similarly, granitic magma saturated with an acidic plagioclase 

 cannot dissolve basic plagioclase but can only react with it and con- 

 vert it into more acid plagioclase. 



These remarks are tantamount to the statement that saturated 

 granitic magma cannot dissolve inclusions of more basic rocks. The 

 magma will, however, react with the inclusions and effect changes 

 in them which give them a mineral constitution similar to that of 

 the granite. These changes will often be accompanied by disin- 

 tegration of the inclusions and the strewing about of the products 

 which may be indistinguishable from the ordinary constituents of the 

 granite. The inclusions may thus become completely incorporated 

 though not in any sense dissolved. It is this action of magmas upon 

 inclusions that makes particularly difficult the problem of distin- 

 guishing xenolith from autolith, i.e., accidental inclusion from cog- 

 nate inclusion. 



Whatever origin one may assign to a granitic magma — let it be 

 formed by differentiation of more basic magma, by differentiation of 

 syntectic magma or by palingenesis of sediments^there seems no 



