402 REGINALD A. DALY 



Velain's observations at the Isle of Reunion led him virtually to state 

 the hypothesis, but, at the time of the publication of his memoir, very 

 little was known as to the actual temperatures of crystallization in 

 lavas, as to the degrees of fluidity which the lavas exhibit at those 

 temperatures, nor as to the density of molten lava, phenocryst, or 

 remelted phenocryst. It was, therefore, impossible for Velain to 

 show the exact conditions under which fractional crystallization can 

 produce an andesitic lava from an original basalt. The experimental 

 studies of the last twenty years have now made it possible to discuss 

 the process of differentiation somewhat more fully. 



It is one purpose of this paper to offer a brief statement of the 

 hypothesis as viewed in the light of the experiments of Doelter and 

 others on the properties of lavas during crystallization. A second 

 purpose is to lay emphasis on the enormous scale in which this particu- 

 lar kind of differentiation of lavas has taken place. It appears to be 

 a world-wide phenomenon. Thirdly, the hypothesis necessarily 

 involves the correlative derivation of certain ultra-basic lavas and 

 rocks from olivine basalt. The conception has thus become of 

 practical value to the writer in helping to explain the recurrent field- 

 association of olivine basalt, augite andesite, and various peridotitic 

 rocks discovered in the Selkirk, Columbia, and Cascade mountain- 

 ranges of British Columbia. The hypothesis will here be presented 

 in a general form, for, while it appears to explain the field-occurrences 

 actually studied by the writer, the conception, like all petrogenic 

 hypotheses, should stand the test of geological experience throughout 

 the world, 



TEMPERATURES AND ORDER OF CRYSTALLIZATION IN BASALT 



As a result of numerous experiments on artificial basic melts and 

 on natural lavas, as observed under the microscope, Doelter has 

 proved that olivine, augite, magnetite, and plagioclase crystallize in 

 the order which has been deduced from the microscopic study of 

 basalt by Rosenbusch, Zirkel, and other systematic petrographers.^ 



According to Doelter, both magnetite and phenocrystic olivine 



crystallize from artificial basic melts at temperatures ranging between 



1200° and 1030° C. The olivine largely crystallizes between 1200° 



I C. Doelter, "Die Silikatschmelzen," Sitzungsberichte der k. Akad. d. Wissen., 

 Vienna, Math.-naturw. Klasse, Bd. 103, 1904, p. 177. 



