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SCIENCE PROGRESS 



mica and quartz are precipitated the residual magma contains 

 a concentration of nepheline molecules with the volatile fluxes 

 appropriate to the formation of cancrinite, hauyne, and sodalite. 

 If such a liquid is squeezed out, or if the quartz and biotite 

 sink away, a foyaite magma remains of the requisite composi- 

 tion demanded by nepheline-syenites and allied alkaline 

 rocks. Intermediate stages, of course, supply alkali-syenites 

 and granites. The following summary of the course of crystal- 

 lisation followed by a basaltic magma when it cools slowly, and 

 under such conditions that the crystals sink, or the residual 

 liquids are drained away, indicates part of a possible, and, 

 according to Bowen, probably the most normal line of descent. 

 At each stage the temperature is lower, and the volatile fluxes 

 are more concentrated, than in the preceding stage. As 

 soon as complete crystallisation is effected the process of 

 necessity stops, and according to the rate of cooling this may 

 take place at any of the specified stages. 



Bowen suggests alternative lines of descent due to decreased 

 activity of the volatile fluxes in early stages, brought about 

 possibly by actual deficiency of these constituents, or by relief 



1 By the separation and individual concentration of pyroxenes, or of plagioclase, 

 pyroxenites or anorthosites may be formed respectively. By more rapid cooling, 

 olivine, pyroxene, and plagioclase may crystallise in close association, giving an 

 olivine basalt or gabbro. 



