128 REGINALD A. DALY 
types of alkaline rocks are not, however, in all cases basic differentiates from 
nephelite syenite magma. The reactions preliminary to the separation of 
quartz and biotite begin at an early stage in the crystallization of basaltic 
magma, and the separation of these minerals may take place at an early stage, 
giving rise to quartz diorites or granodiorites. The possibility of the formation 
of alkalic magma at a stage much earlier than the biotite granite stage is 
thereby introduced if conditions are favorable. Favorable conditions seem to 
consist in the opportunity for sinking, not only of the plagioclase crystals and 
femic minerals, but also of the quartz crystals in sufficient amount. Thus may 
result relatively ‘“‘basic”’ alkaline magmas from which such rocks as basanite 
might be formed, and nephelite syenite itself as a light differentiate. 
Assumed desilication by the settling-out of quartz crystals—The 
evolution of feldspathoid-bearing magma from the biotite-granite 
stage is supposed to be accomplished by the separation of crystals 
of orthoclase, albite, mica, and quartz. A critical element is the 
actual removal of quartz, and the doubtful nature of this assump- | 
tion affects the whole argument. In frozen granite, whether a part 
of a large batholith or wholly constituting a more quickly chilled 
sill or dike, the quartz is all, or nearly all, interstitial. In the typical 
case there is no evidence that the quartz crystallized early, so as 
to be capable of settling-out. Rhyolites do bear phenocrystic 
quartz, but probably for special reasons. It would be wrong to 
regard rhyolites as exact homologues to granite in mode of crystalli- 
zation. In any case the groundmass of rhyolite or quartz porphyry 
is itself very rich in free silica, and it is difficult to imagine how the 
groundmass quartz could be removed so as to leave a quartz-free 
residuum like nephelite syenite. 
The same trouble arises in connection with the second postulated 
method of development, namely, from quartz-diorite or granodiorite 
magma. The quartz of quartz-diorite and granodiorite is again 
interstitial and: seldom shows any tendency to phenocrystic 
relations. 
Origin of basanite——Can basanite be derived by the sinking of 
quartz and other minerals from evolving basalt? Solid basanite 
has a specific gravity near 2.95. In the liquid state its specific 
gravity would be about 2.60, a value little if any greater than that 
of the magma in the immediately preceding stage. On the other 
hand, the quartz crystals at the temperature of this magma would 
