4o6 REGINALD A. DALY 



Each of these glasses would expand, with heating, at least as fast as 

 diabase, e.g., 3 per cent, for 1200° C. (Barus). At 1200° C, therefore, 

 the remelted crystals would have specific gravities at least as low as 

 the values shown in the third column. The specific gravity of normal 

 basalt at 1200° C. for a type which is holocrystalline at specific 

 gravity of 3.00, is about 2.74; that for a type holocrystalline at 3 . 10 

 is about 2.83 — both these values being calculated from the data of 

 Barus and Douglas. There seems to be good reason "to believe, 

 therefore, that the remelted and more or less perfectly dissolved 

 phenocrysts would not sink indefinitely deep in the lava column, but 

 would come to rest, forming one or more ultra-basic layers in the 

 conduit. 



In an active volcano the time allowed for the growth and 

 sinking of phenocrysts may be long enough for a complete differ- 

 entiation, or it may suffice only to remove some of the olivine and 

 magnetite from the cooling surface layer of the column, or it may 

 be so short as to forbid the growth of phenocrysts in the vent. Erup- 

 tion will necessarily arrest or greatly retard the process. Where the 

 outflow is rapid and continuous the original olivine basalt appears at 

 the earth's surface. There, of course, the rapid cooling generally 

 prevents recognizable differentiation in the way possible, and, appar- 

 ently necessary, in the vent itself when the basalt stands within it for 

 a considerable time. 



We have, then, to expect in nature a continuously graded 

 series of lavas from pure olivine basalt, through olivine-free basalt, 

 to those phases of the mother-liquor which must approximate 

 a basic augite andesite and then an acid augite andesite. The 

 last rock would thus represent the one phase, the more voluminous 

 phase, of this kind of differentiation. In view of the notably uniform 

 composition of olivine basalts throughout the world we must further 

 expect, that, in all cases where the fractional crystallization has run 

 a complete course, the more acid phase should be relatively imiform 

 in chemical composition. Its phenocrysts form when the magma's 

 viscosity is relatively high and sinking is very slow. 



The other products of the differentiation must also show a very 

 great variation in composition. According to the special thermal 

 conditions and shape of each lava-column, the phenocrysts must sink 



