LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 91 



all other igneous rocks could be derived from basaltic magma by 

 differentiation alone. Incidentally the necessity for assuming a 

 granitic shell of the earth of totally different origin from all later 

 granites is eliminated. Complete accord with Daly is, however, 

 reached with regard to the importance of gravitative differentiation 

 and the importance of restriction of differentiation at the contact 

 in the production of contact phases, commonly, though not always 

 basic phases. 



In conclusion, the writer desires to express his indebtedness to 

 many of his colleagues at this laboratory, physicists and chemists as 

 well as petrologists, for helpful discussion of some of the problems 

 treated. 



