90 JOURNAL OF GEO LOGY SUPPLEMENT 



diopside and quartz from a quartzite-dolomite terrane, for example. 

 On the contrary, it is believed by the great majority of petrolo- 

 gists that the rocks of any area vary among themselves in a syste- 

 matic manner which indicates derivation from a common, stock 

 through some systematic process of differentiation from that stock. 



The decision is reached that this differentiation is controlled 

 entirely by crystallization. The sinking of crystals and the 

 squeezing out of residual liquid are considered the all-important 

 instruments of differentiation, and experimental evidence is 

 adduced to show that under the action of these processes typical 

 igneous-rock series would be formed from basaltic magma if it 

 crystallized (cooled) slowly enough. The characteristic occurrence 

 of basaltic magma as regional dykes and as the material of the great 

 fissure eruptions is considered evidence of the primary nature of 

 basaltic magma. It is concluded, therefore, that most, if not all, 

 igneous rocks have probably been derived from basaltic magma, 

 the processes of differentiation that have been pointed out above 

 emphasizing the lighter, more salic and alkalic differentiates in the 

 upper portions of very large, slowly cooled bodies. 



In some of its more fundamental aspects the conception of the 

 igneous rocks reached is, therefore, closely related to that advanced 

 by Daly. Basaltic magma is the primary material of all post- 

 Keewatin igneous action according to Daly. The more immediate 

 derivatives of this magma, peridotite and augite andesite, he con- 

 siders to be formed through gravitative differentiation, which he 

 formerly believed to be due to the sinking of crystals, though he 

 now leans toward the formation of immiscible liquid portions. For 

 the formation of more remote derivatives, diorite, granodiorite, 

 granite, nephelite syenite etc., however, he considers that in each 

 case assimilation of a special kind of foreign material, the acid shell 

 of the earth or various t3^es of sediments, must first be accom- 

 plished and has therefore introduced the idea of stoping with 

 abyssal assimilation or assimilation at unobservable depths. 

 Here the present conception diverges radically. The reasons for 

 assuming abyssal assimilation, or any kind of assimilation on the 

 scale advocated by Daly, are considered to be entirely removed 

 when it is shown that the t3/pes enumerated above and probably 



