LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 6i 



differentiation, with leucite as one of the crystalline units in that 

 process might give rise to rocks comparatively rich in leucite. The 

 relative abundance of leucite-bearing t3Apes as effusive and hypabys- 

 sal rocks as compared with their rarity as truly abyssal rocks lends 

 support to the idea that near-surface conditions favor their forma- 

 tion. Such considerations raise the whole question of the extent 

 to which evaporation, on the one hand, and cooling on the other, 

 have controlled the crystallization of a given rock mass. 



Under the foregoing conception of the leucite rocks they should 

 presumably belong at a late stage of the genetic sequence, a stage 

 appropriate to the preliminary formation of biotite. This appears 

 to be true of the lavas of the Aeolian Isles as determined by Bergeat, 

 who refers to the leucite types as an old-age manifestation following 

 after a sequence exhibiting increasing acidity.^ 



ESCAPE OF THERMAL WATERS 



The crystallization of the constituents of the nephelite syenites 

 and related rocks is not the end of the process of crystallization 

 from the magma. The precipitation of analcite and other zeolites, 

 apophylite, and thomsonite follows thereafter and the process 

 passes on into the stage considered to belong rather to that of 

 thermal waters than to magmas. In these waters there is an 

 increasing concentration of the very soluble salts of the alkalis such 

 as NazS, NaaCOj, NaCl, etc. (see equilibrium reactions, pp. 44 and 

 45), and they probably constitute the alkaline ascending waters of 

 primary ore deposits. These waters may, of course, be expelled 

 from the magma at a much earlier stage, say the granitic or grano- 

 dioritic, if the cooling is at such a rate that differentiation is com- 

 pleted at that stage. 



OTHER CONCEPTIONS OF THE ALKALINE ROCKS 



Daly has offered a theory of the alkaline rocks in general, which 

 supposes that they are differentiates from subalkaline magmas that 

 have absorbed limestone. The formation of feldspathoids is con- 

 sidered due to a desilication of the magma consequent upon the 



^ A. Bergeat, Compte-reiidu, XII™** session, Congres Geologique International, 

 Canada, p. 250; "Die aolischen laseln," Abhand. der k. bayer. Akademie der Wiss., 

 II, Kl, XX, (Munich, 1899), p. 270. 



