Evolution of Igneous Rocks. 471 
granite. This is explained as ‘‘ the result of an increasing concentra- 
tion of the volatile constituents”, which lowers the temperature of 
consolidation and allows the formation of the water-bearing minerals, 
biotite and hornblende." 
The origin of the alkaline rocks is traced to the influence of the 
water, fluorine, and other mineralizers. During the crystallization 
of biotite-granite, if the residual liquid be separated in some way 
from the already formed biotite and quartz, it must have a greatly 
increased concentration, not only of such volatile substances as H,O, 
Cl, $03, CO,, and so forth, but also of such alkali silicates as albite, 
orthoclase, nephelite, and kaliophilite; and the latter molecules 
may combine with the former to give the felspathoids. Thus the 
separation of the alkali-syenites may occur at any of the stages at 
which the formation of biotite and quartz is possible, that is, at any 
stage between diorite and granite. While this line of descent results 
from the maximum activity of the volatile constituents, there is an 
alternative which, in its earlier stages at least, represents the result 
of the minimum activity. If a gabbro magma be impoverished in 
water and similar constituents, the formation of biotite is inhibited, 
and the proportion of such silicates as orthoclase increased, so that 
the latter separates out along with lime-plagioclase, giving rise to 
an essexite or similar rock. Even in this case, however, the ultimate 
differentiate may be nephelite-syenite, if at any stage there is 
a concentration of the mineralizers. Thus we have the two alternate 
lines of descent of the alkaline rocks, the intermediate stages between 
gabbro and nephelite-syenite being, in the one case diorite and 
normal syenite, and in the other essexite and augite-syenite. 
Leucite-bearing rocks are supposed to be formed in a similar 
fashion, and the fact that this comparatively rare mineral is practically 
confined to hypabyssal and effusive rocks is considered to be due to 
the facilities, which the latter often have, for the escape of 
-mineralizers, thus allowing the potassium salts to consolidate as 
leucite instead of the more complex minerals such as mica. 
The fact that the theory has a sound experimental basis renders it 
less liable to criticism than most previous attempts. It must be 
remembered, however, that it is only the early stages, namely, the 
derivation of ultrabasic rocks on the one hand and of the diorite and 
certain syenites on the other, from the primary basaltic magma, that 
has been subjected to verification in the laboratory. Experimental 
difficulties have hitherto precluded any systematic investigation of 
water-bearing magmas, and hence comparatively little is known 
concerning the formation of the micas and other hydrated minerals. 
_ Recent work has demonstrated beyond all doubt the practical absence 
of eutectics in basaltic rocks, and has shown that the presence of 
solid solution between two or more of the constituents increases the 
variation in the possible results of the consolidation. Hence it is 
very unsafe to prophesy the results, of crystallization where the 
degree of solid solution is immensely greater, as in the case of the 
micas and the amphiboles. Owing to this complex isomorphism, 
1 Cf. Allen & Clement, Amer. Journ. Scz., ser. IV, vol. xxvi, pp. 101-18, 1908. 
