56o N. L. BOWEN 



liths are not simply melted xenolith but essentially normal igneous 

 material of a late stage of the reaction series. The reaction-rim 

 stage is a temporary one except in so far as it may be preserved 

 about some inclusions by exhaustion of the magma. Others dis- 

 appear entirely by diffusion of the rim material into the magma and 

 distribution (with possible precipitating effects) of any surplus 

 material. Thus the liquid is pushed onward in the reaction series, 

 not only through addition of the rim material, but also because this 

 necessitates some precipitation of the early-formed minerals from 

 the basic magma. Further fractional crystallization may therefore 

 give differentiates identical with, or closely related to, the reaction- 

 rim material, but normal differentiation might have given it also. 

 Inclusions of some acid sediments, richly charged with volatile 

 substances (mainly water) and immersed in a basic magma under 

 conditions permitting their retention of this volatile matter, might, 

 theoretically, be converted to liquid in toto. Actually, few inclu- 

 sions show any such effect, from which fact it may be assumed that 

 the conditions mentioned are seldom realized. That the reaction 

 effect discussed is the important one quantitatively sedms unques- 

 tionable. 



EFFECTS OF GEANITIC MAGMA ON INCLUSIONS OF 

 SEDIMENTARY ORIGIN 



In discussing the reaction of magmas with inclusions we have, 

 in the case of basaltic magma, made some reference to the super- 

 heated condition. Daly points to basaltic magma as the heat 

 bringer, and has presented evidence that such magma enters into 

 igneous-rock economy on a different basis from all other magmas."" 

 If this be true, and his reasons seem to me convincing, basaltic 

 magma is the one magma that may, presumably, be assumed to have 

 superheat on some occasions. All other magmas, whether they may 

 be formed by differentiation of basaltic magma or by differentiation 

 of syntectic magma must usually be saturated, unless it be that 

 locally, at volcanic vents, a special source of heat is available. This 

 possibility has little quantitative petrogenic significance and it is 

 perhaps a realization of the commonly saturated condition of other 



^ R. A. Daly, Igneous Rocks and Their Origin, p. 458. 



