GENESIS OF THE ALKALINE ROCKS A 
between the syenite and shonkinite of the Square Butte laccolith, 
nor of the apparent homogeneity of that shonkinite. It fails to 
take account of the lack of ultra-femic phases in the Pigeon Point 
and other sills which display notable salic differentiates. Its con- 
sequence, that all monomineralic rocks except certain sulphides are 
crystal “‘rafts,”’ may fit the case of anorthosite, but not other cases. 
Other special objections are found in the chemical nature of quartz 
diabase, basanite, and the ‘‘alkaline basalts.” The absence of 
foyaitic phases in, or apophysal from, most large bodies of granite, 
granodiorite, and quartz diorite does not agree with the theory as 
developed. Similarly quartzose lavas, expected on the theory, are 
not found in many volcanic piles containing trachyte or phonolite 
with basalt. The lack of quartzose lavas in the enormous and 
therefore long-lived volcanoes distributed over the main ocean floor 
is a fact not easily explained by the theory. 
Bowen has given the clearest, most detailed argument for the 
significance of fractional crystallization which has yet been pub- 
lished. His general theory takes cognizance of: the proved genetic 
association of alkaline rocks with subalkaline magmas, especially the 
basaltic; the small size of alkaline bodies; their richness in gases 
and rare elements; the consequent effects regarding grain and 
‘variability of composition; the roof positions of alkaline differ- 
entiates in many sills, laccoliths, and batholiths; and the con- 
centration of alkalies. Nevertheless, all of these facts are explicable 
also on the syntectic-differentiation hypothesis, which does not 
encounter certain difficulties facing the theory of pure fractional 
crystallization. 
GENERAL CONCLUSION 
Thus the study of the geological publications issued since the 
completion of the manuscript of the writer’s Igneous Rocks and 
Their Origin has led him to renewed faith in the general explanation 
there advanced for most of the alkaline rocks. Several expert field 
observers have sympathetically entertained the hypothesis of 
control by the syntexis of basic sediments charged with volatile 
matter. Evidence for the derivation of alkaline rocks from sub- 
alkaline magmas has been still further accumulated. Some authors 
have expressed a degree of confidence that basalt is the only primary 
