222 C. JV. Fennev — Crystallization of a Basaltic Magma 



The phenocrysts were probably formed during rather slow 

 cooling in the flowing mass of lava, and the bordering micro- 

 litic growths represent the continuation of the crystallization 

 during the initial stages of chilling, before increasing viscosity 

 put a stop to the process. Their brown color seems best 

 explained by the separation of magnetite dust at the same time 

 that diopside and plagioclase were crystallized out. If this is 

 true, the eutectic ratio had been reached, and if normal crystal- 

 lization had been followed the continuation of the process would 

 have taken the form of simultaneous growth of crystals of 

 diopside, plagioclase, and magnetite. This was prevented by 

 the increasing viscosity of the liquid, with which crystalliza- 

 tion could not keep pace. 



The almost simultaneous appearance of diopside and plagio- 

 clase at the initiation of crystallization, attended or followed 

 after only a short interval by magnetite, would appear to 

 demonstrate that the composition of the magma as erupted did 

 not differ greatly from that required to form a eutectic of the 

 three. 



The only other feature of interest in the slide is the presence 

 of a number of olivine crystals, which have been strongly 

 attacked and resorbed by the magma. This phenomenon will 

 be taken up later. 



Slide No. 1 is almost a duplicate of No. 105 and exhibits the 

 same set of phenomena, except that there is no evidence of 

 olivine grains, and that the glassy groundmass shows with 

 high magnification abundant brown dust. Diopside pheno- 

 crysts are a very minor constituent but plagioclase is abundant. 



In No. 57 the chilling was apparently a little less sudden, for 

 the phenocrysts have a tendency to blend with the plumose 

 microlitic borders and a smaller proportion of the slide is 

 occupied by strictly undifferentiated glass. 



Slides 30, 85, and 90 are other examples showing essentially 

 the same relations of phenocrysts, microlites, and undifferen- 

 tiated glass. In No. 132 (fig. 3), the microlites are seen to 

 be spreading over a greater portion of the field, and in No. 2 

 a further stage is shown. 



This latter slide was taken from the same hand specimen as 

 No. 1, but whereas the portion of the rock from which No. 1 

 was taken presented a decidedly vitreous appearance, that from 

 which No. 2 was prepared was a little farther removed from 

 the chilled crust and was of a felsitic character. Microscopic 

 examination confirms the deduction which would naturally be 

 drawn. In No. 1 the phenocrysts are merely bordered by 

 plumose microlites and a large part of the section is undifferen- 

 tiated glass. In No. 2 the microlites have spread over the 

 entire held not occupied by the phenocrysts and no glass can 



