74 JOURNAL OF GEOLOGY— SUPPLEMENT 



cosmogony, which subject it is desired to avoid in this paper. It 

 may be noted, however, that the existence of a source of basaltic 

 magma does not necessarily imply a Hquid layer and possibly not 

 even a basaltic layer, since it is conceivable that disseminated 

 basaltic hquid might have been generated as occasion demanded 

 by some sort of selective fusion of heterogeneous non-basaltic 

 material. This suggestion is not offered as an expression of 

 opinion, but merely as an indication that belief in the continuous 

 existence of a source of basaltic magma does not necessarily force 

 one to prefer either the view that the earth was once molten or the 

 view that it has always been a relatively cold body. Until the 

 petrologist finds in his own field some reason for advocating the one 

 or the other view, there is little need of his expressing a preference. 

 Neither is it necessary for the petrologist to express an opinion as 

 to the physical condition of the earth's interior, whether magmas 

 come from great depth or moderate depth, whether they are gener- 

 ated as a result of some mechanical production of heat or have 

 persisted from a once-molten earth. He may reasonably start 

 with magma, however produced, and with such a magma or 

 magmas as the geologic evidence appears to dictate. 



THE NORMAL LINE OF DESCENT 



The facts indicating the parental nature of basaltic magma have 

 been detailed in the foregoing. As already stated, they are not 

 considered as proving that basaltic magma plays this role, but 

 rather as constituting a strong plea for the inclusion of this idea 

 among the petrologist' s working hypotheses. Especially is this 

 desirable when a conception is gained of a process whereby igneous- 

 rock series can be derived from basaltic magma. This process of 

 evolution, through the action of crystallization differentiation, has 

 already been stated in detail, with emphasis, for the most part, 

 on its physico-chemical aspects. Consideration will now be given 

 to some of its geological aspects. 



In the following an outline of the conception of the igneous rocks 

 here advanced is presented. It may serve as a summary of the 

 fuller discussion presented in the preceding pages. Being in 

 schematic form, it is necessarily somewhat rigid and is not to be 

 interpreted without due reference to the discussion in the text. 



