LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 17 



resulting great viscosity may be reasonably expected to prevent 

 important sinking of crystals/ Nevertheless the sinking of crystals 

 may be of considerable importance in a very large body of salic 

 magma which exposes to the surrounding rocks a minimum of sur- 

 face in proportion to its mass. 



The two processes involving relative movement of crystals and 

 liquid — the sinking of crystals and the squeezing out of residual 

 liquid — aid each other in a general way in the production of an 

 arrangement of the various differentiates, such that the heaviest 

 lies at the greatest depth, and the lighter ones at lesser depth — 

 a gravitative adjustment. 



Daly has collected from the literature and from personal study 

 a host of instances illustrating quite convincingly the fact of gravi- 

 tative adjustment.^ The two processes mentioned above are, the 

 writer believes, the sole instruments of its production. On a later 

 page a discussion will be given of the working of the processes in 

 specific cases. 



The formation of zoned crystals is a process which may well be 

 discussed at the same time. It is somewhat related to the two 

 processes mentioned above, inasmuch as the zoning of a crystal 

 effectively separates the earlier-formed part of the crystal from the 

 neighboring liquid in which it formed. This fact has an important 

 effect on crystalHzation and therefore on differentiation. 



THE ASSOCIATION OF DIABASE AND MICROPEGMATITE 



Diabases with micropegmatite interstices are very common. 

 Sometimes the micropegmatite (granophyre) is separated as a 

 distinct body, a granite, granodiorite, or quartz diorite in composi- 

 tion. This association is of fundamental importance to petrogenic 

 theory and will be made the starting-point for a discussion of the 

 geologic evidence supporting crystallization differentiation. It is, 

 in many cases, clearly shown that when the diabasic (basaltic) 

 magma was intruded as a small body and was therefore quickly 



' In pegmatite sheets in California in which, no doubt, fluidity was maintained, 

 Schaller has found very striking examples of the sinking of garnet crystals (personal 

 communication from Dr. Schaller). 



2 Op. ciL, pp. 228 f. 



