598 JV. L. Bowen — Melting Phenomena of the 



account of the possibility that the liquid may be separated 

 from the crystals, say by the sinking of the latter or by rein- 

 trnsion of the former to a higher level. The actual tempera- 

 tures concerned will, of course, be much lower in the natural 

 rocks. 



The reversal* of the order of zoning sometimes noted in 

 natural plagioclase may perhaps result from various causes. 

 That undercooling might bring about a reversal may be seen 

 from inspection of fig. 1. If liquid of composition Ab a An x 

 were cooled quickly to 1280°, rapid formation of crystals 

 (AbjAnJ might take place at this temperature, and the heat 

 (latent) released might quickly raise the temperature of the 

 whole mixture to 1300°, at which temperature a zone of crys- 

 talline material of composition Ab 45 An 55 would form around 

 the earlier less calcic crystals. Similar undercooling might 

 bring about a like result in a natural magma. 



The secret of reversal may at times be found in the presence 

 of other lime-bearing minerals in the natural rock. Move- 

 ment of the magma during crystallization might bring about 

 the same result by exposing the crystals to liquid of composi- 

 tion different from that out of which they had been crystalliz- 

 ing. To such movements also, and perhaps, at times, to sudden 

 lowering of temperature, may be attributed the sharp bound- 

 aries that various zones often exhibit. 



In view of the very great quantitative importance of the 

 plagioclases in igneous rocks, it is a matter of some satisfac- 

 tion to find them obeying the laws of physical chemistry to the 

 extent here found. 



Summary. 



The method of quenching was applied to the determination 

 of the melting intervals of pure, artificial plagioclase feldspars. 

 It was found possible to determine accurately the temperatures 

 of beginning of melting (solidus) for compositions ranging 

 from pure An to Ab 3 An 1 , and of completion of melting 

 (liquid us) for the range An — AbgAn^ Very pure natural 

 material, Bakersville oligoclase, was used to determine the 

 point on the solidus corresponding to its composition. Similar 

 material, Amelia County albite, served to fix the melting point 

 of albite. The results of this work are summarized at the end 

 of Table II and diagram matically expressed in fig. 1. 



It was also found possible in several instances to determine 

 the composition of the liquid phase, present at temperatures 

 within the melting interval, by measuring the refractive index 

 of the quenched glass. Thus points on the liquid us were 

 determined by an independent method and were found to be 



*N. L. Bowen, J. Geol., xviii, p. C62, 1910. 



