548 N. L. BOWEN 



rock-forming oxides. Obviously the effects of all possible sedi- 

 ments cannot be examined, but our purpose will be served if we take 

 the most extreme departure from igneous composition. As repre- 

 sentative of this condition, for quartzite we may imagine the addi- 

 tion of pure quartz; for limestone, of pure calcium carbonate; and 

 for shale, of pure kaolin. Any actual sediment would usually con- 

 tain all of these, together with other constituents that lessen its 

 departure from igneous composition. 



Let us take a magma of basaltic composition which, on crystal- 

 lization with comparatively rapid cooling, would form mainly plagio- 

 clase, and clinopyroxene, with some olivine, a little ore mineral and 

 possibly some orthopyroxene. All of these are minerals of variable 

 composition; some of them, in particular the pyroxene, vary with 

 respect to several components and to this is to be attributed the 

 fact that the number of solid phases formed is less than the number 

 of oxides present. This fact permits particularly wide adjustments 

 in the composition of the solid phases without the appearance of 

 new ones. Such basaltic magma, with a little superheat, could 

 directly dissolve a moderate amount of sediments, yet even if these 

 were of extreme composition the magma would crystallize with the 

 production of the same solid phases as those mentioned above if 

 crystallized under the same conditions. 



Normally only saturated magma would be available and the 

 superheated magma mentioned above would rapidly become satu- 

 rated as a result of solution of inclusions. For the case of such 

 saturated magma it may be stated as a first principle that the sedi- 

 ment would, in so far as its composition permitted, tend to be con- 

 verted into the phases with which the magma is saturated. And 

 the material necessary for such changes in the sediment would not 

 be merely subtracted from the liquid but adjustments of the com- 

 position of the liquid would occur through separation of further 

 amounts of the phases with which the liquid is saturated. 



The precise changes in the composition of the solid phases formed 

 cannot be represented graphically on account of the number of 

 components involved, but equations can be written that afford a 

 generalized conception of the possible adjustments for the addition 

 of calcite, silica, and kaolin respectively. 



