252 /. E. L. VOGT 



than usual with the plagioclase of the deep-seated rocks may perhaps 

 be due to the acid composition of the magma, which at a somewhat 

 advanced stage of the crystallization may have been rather viscous. 

 The great difference in equilibrium between the solid and the 

 fluid mix-crystal components, in dikes, and flows, with very incom- 

 plete, but as a rule not absolutely lacking equilibrium on one hand, 

 and deep-seated rocks in some cases with medium, though mostly 

 with practically complete equilibrium, on the other hand, must 

 be due to the great difference of the time of the crystallization. 



H. E. Boeke states in his Grundlagen der phys.-chem. Petrographie 

 ([1915], p. 99) that: "Diese, eine Diffusion im festen Zustande 

 erforderliche Anderung findet namentlich bei Silikaten nur mangel- 

 haft statt." This is so far correct since this diffusion goes on much 

 more slowly in silicates than in metals, but the diffusion in the 

 silicates may become very extensive or even complete, when, as 

 in many deep-seated rocks, there was sufficient time and the magma 

 was rather thin fluid. 



[To be continued] 



