LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 41 

 THE IMPORTANCE OF THE FORMATION OF BIOTITE 



Considerable importance has been attached in the foregoing to 

 the early separation of olivine, but there are many cases of diabases 

 with micropegmatite interstices, or even a separate salic differen- 

 tiate, where there is good evidence that no early separation of 

 olivine took place. Whence, then, came the free silica (quartz) ? 

 It is, of course, possible in some instances that the magma as 

 intruded was a quartz-diabase magma; yet in many cases the 

 quickly cooled, small dykes show no quartz, and prove clearly that 

 the magma was of normal, basaltic type. There must be some pro- 

 cess which results in the separation of free silica as quartz entirely 

 independent of the separation of olivine in some cases, supplementing 

 it in others. This process is that involved in the separation of the 

 mica, biotite. The femic material which separates as pyroxene from 

 the more basic diabase, and dominantly as hornblende in rocks of 

 intermediate composition, appears in solid solution with alkahc 

 molecules as biotite-mica in the salic differentiates. It is fre- 

 quently noted, moreover, that when the salic material occurs 

 rather as interstitial micropegmatite, the micropegmatite liquid 

 where it bordered against pyroxene has reacted with it to form 

 biotite. This difference between the natural magma and the 

 artificial melts in which a diopsidic granite was the salic differ- 

 entiate is plainly the outcome of the fact that the artificial melts 

 are anhydrous whereas the magma contams various volatile con- 

 stituents. The formation of hornblende and still more of the micas, 

 with their essential content of water and often of fluorine, is the 

 result of an increasing concentration of volatile constituents. 



The change in the nature of the f erromagnesian constituent with 

 the progress of crystallization has not been investigated experi- 

 mentally on account of the great difficulty of dealing with iron- 

 bearing minerals. The principles involved are, however, well 

 illustrated in certain investigated systems and will be pointed out 

 before proceeding to a discussion of the processes taking place. 



EQUILIBRIUM IN SILICATE LIQUIDS 



It is sometimes possible to obtain, from a study of the crystal- 

 lization of certain mixtures, definite evidence of equilibrium 

 reactions in the liquid. 



