ORIGIN OF AUGITE ANDESITE 



407 



to different depths, and be segregated or dissolved in highly different 

 proportions in different levels of the lava-column. From the original 

 olivine basalt many types of ultra-basic basaltic magma and of peri- 

 dotitic magma might be developed in the same conduit. During 

 energetic eruption or intrusion into the walls of the conduit these 

 might become mixed with each other and the resulting rocks present 

 just such great variation as is actually observed in the peridotite 

 family. Many peridotites, the picrites, limburgites (magma basalts), 

 and abnormally olivinitic basalts are, in this view, the rocks derived 

 from the fractional crystallization of olivine basalt, while augite 

 andesite represents the other pole of the differentiation. 



TESTS OF THE HYPOTHESIS 



I. Chemical relations. — The view that olivine basalt may be the 

 parent of augite andesite and of several ultra-basic igneous types is 

 well supported by a comparison of Streng's total analysis of a dolerite 

 and his analysis of its own glassy base.^ The two are here quoted. 



SO2.. 

 TiO^ 

 AI2O3 

 FejO, 

 FeO.": 

 MgO. 

 CaO. 

 Na^O 

 K,0. 

 H2O. 



Dolerite 



49.08 

 1.82 



13-43 

 6.49 



5-92 

 9-58 

 8.92 

 3-42 

 1. 00 

 0.32 

 0.51 



100.49 



Glassy base 



99-77 



The phenocrysts of the dolerite include andesine, augite, enstatite, 

 and olivine; and magnetite is, of course, a noteworthy constituent. 

 The composition of the glassy base clearly tends toward that of an 

 augite andesite, though the whole possible amount of phenocrystic 

 development was, in the case of this dolerite, not attained. Streng 

 observed that the phenocrysts were largely or altogether wanting in 

 the upper-surface layer of the dolerite flow, but he thought it " improb- 

 able" that their absence was due to settlement of the crystals. He 

 attributed the phenomenon rather to the operation of Soret's principle, 



I Neiies Jahr. fiir Min., etc., 1888(2), p. 211. 



