ESSAY-REVIEWS 69 



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

 a gravitative adjustment." 



Actual experiments with various melts have shown that 

 the crystallisation of basaltic magma, if slow, provides a con- 

 tinuous variation from basalt to syenite. The early formation 

 of olivine enriches the residual liquid in silica ; similarly, the 

 crystallisation of calcic plagioclase allows a concentration of 

 albite, and that of enstatite an enrichment in diopside — pro- 

 vided, of course, that these minerals sink, and so are not 

 resorbed at the level where they separated. If sufficient silica 

 were liberated, the residual magma might pass to one of 

 granodiorite or even granitic composition, and this is considered 

 to be the explanation of the frequent intimate association of 

 dolerite and granophyre. It is clear, however, that the early 

 separation of olivine is inadequate to supply all the free silica 

 required by a granite. The discrepancy is met by an appeal 

 to biotite, a mineral which characterises by far the majority of 

 granites, and which represents considerable proportions of the 

 alkalies and the ferro-magnesian constituents in their least 

 siliceous combinations. " This difference between the natural 

 magma and the artificial melts, in which a diopsidic granite 

 was the salic differentiate, is plainly the outcome of the fact 

 that the artificial melts are anhydrous, whereas the magma 

 contains various volatile constituents. 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 in- 

 creasing concentration of volatile constituents." The slow 

 crystallisation of a basaltic magma leads to a concentration of 

 orthoclase, albite, and water, and the effect of the latter is to 

 bring about reactions such as : 



KAlSi 3 8 $ KAlSi0 4 + 2Si0 3 

 NaAlSi 3 8 $ NaAlSiO* + 2Si0 2 



The orthosilicates then combine with ferromagnesian silicates 

 to form biotite, which is precipitated with free quartz. The 

 more significant molecules that in the presence of water and 

 other volatile fluxes become concentrated in the residual 

 liquid are the alumino-alkaline orthosilicates, and the ferro- 

 magnesian orthosilicates (with gradual elimination of mag- 

 nesium and consequent further enrichment in iron). 



Bowen now goes a step further and suggests that when 



