LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 23 



liquid as the temperature falls, until the last drop is used up, and a 

 concomitant change in the composition of the mix-crystals. Con- 

 trast such crystallization with the crystallization in a simple 

 eutectic system, which is sometimes regarded as throwing light on 

 the crystallization of igneous rocks. In the eutectic system, in all 

 possible mixtures, the composition of the liquid continually 

 approaches that of the eutectic mixture, and when it attains this 

 composition there is no further change and no change in tempera- 

 ture until all is crystalline. The temperature of final crystallization 

 is the same for all mixtures. The difference between these two 

 types of systems is due principally to the existence of a mix-crystal 

 series in the one and its absence in the other. Through considera- \ 

 tion of eutectic systems, little aid is to be obtained toward the 

 understanding of the crystallization of igneous rocks, in which 

 mix-crystals are so common. 



If the composition of the original liquid was that of the point D, 

 forsterite would crystallize first as before and crystallization would 

 follow the same course as in the two preceding cases {M and P) until 

 the temperature of the point F is reached. At this temperature 

 the re-solution of forsterite is complete and the mixture consists of 

 liquid of composition F and pyroxene of composition R. The 

 composition of the liquid now leaves the boundary curve and crosses 

 the pyroxene field on the curve FE, pyroxene continuing to crystal- 

 lize and changing in composition toward S. When the temperature 

 of the point E is reached, cristobalite begins to crystallize. At this 

 temperature the liquid has the composition E and pyroxene the 

 composition S. With further lowering of temperature the com- 

 position of the liquid changes along the boundary curve from E 

 toward Z, cristobalite and pyroxene continue to crystallize, and the 

 pyroxene changes in composition toward T. At the temperature 

 of the point Z, cristobalite changes to tridymite, and with further 

 lowering of temperature tridymite and pyroxene continue to crystal- 

 lize. When the temperature of the point G is reached, the liquid 

 finally disappears; the last minute quantity has the composition G 

 and the pyroxene has the composition T. The whole now consists 

 of pyroxene of composition T and tridymite in the proportion 

 pyroxene : trid3nnite = DX : DT. 



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