ORIGIN OF AUGITE ANDESITE 405 



will continue during the time occupied in the drop of temperature 

 of another 100° C. or to the point when practical rigidity is established ; 

 but the rate of settlement must then be very considerably slower than 

 in the interval, 1200° — 1050° C. During the last-mentioned interval 

 practically all of the olivine, much of the phenocrystic augite, and 

 much of the magnetite of early generation has settled out of the 

 upper part of the lava-column. Below 1050° C, the mother-liquor 

 crystallizes. New crystals of magnetite and phenocrysts of augite 

 and feldspar are formed but, because of the greatly increased vis- 

 cosity, do not sensibly sink or rise in the freezing melt. 



To estimate the chemical composition of the mother-liquor it 

 would be necessary to know the composition of the original basalt 

 and the quantity and composition of each settled-out phenocrystic 

 material. The problem may be solved through a careful quantitative 

 chemical study of a typical oh vine basalt; the results are of the same 

 order as those obtained from a comparison between the average com- 

 positions of the world's olivine basalt and of its phenocrysts. The 

 result of these comparisons will be detailed on a later page. 



FORMATION OF ULTRA-BASIC MAGMAS AND OF AN ANDESITIC 

 MOTHER-LIQUOR 



As the phenocrysts sink they enter levels in the conduit where the 

 temperature is higher than near the surface, and where the basalt 

 is as yet completely molten. Probably the lava-column grows slightly 

 more dense toward its base, according to faint chemical differences 

 in the successive strata. The sunk phenocrysts, at the lower levels 

 and higher temperatures will, in turn, become re-dissolved or melted. 

 Deville,' Doelter^ and others have shown that both olivine and 

 augite expand extraordinarily in passing from the crystalline state 

 to the glassy. The specific gravities of a few typical specimens at 

 room temperatures are indicated in the following table : 



» Cf. F. Zirkel, Lehrbuch der Petrographie (Leipzig, 2d ed., 1893), Vol- ^' P- ^80. 

 2 C. Doelter, Physikalisch-chemische Mineralogie (Leipzig, 1905), p. 8. 



