GENESIS OF THE ALKALINE ROCKS 11g 
lying, more salic phase. For example, the lower four-fifths of the 
Pigeon Point sill on the shore of Lake Superior is made up of gabbro. 
Locally the gabbro does bear interstitial micropegmatite, but from 
below upward there is no regular increase in this more siliceous 
ingredient. Such increase is rapid in the thin, overlying, inter- 
mediate rock, which is in turn overlain by the thicker, nearly 
homogeneous roof phase, micropegmatitic granite or red rock. 
According to the theory under discussion, the quartzose red rock 
represents the last surviving liquid of basaltic magma from which 
a correspondingly large crop of femic crystals had settled out. At 
some level between roof and floor these should be specially con- 
centrated. Yet nowhere in the sill is there any important con- 
centration of olivine, pyroxene, or iron oxides beyond the amounts 
characterizing a normal gabbro. The hypothetical ultra-femic 
phase is missing, despite the field evidence that the original magma 
was a rather typical gabbro." 
The same is true for at least some of the magnificent sills of 
the Purcell Mountains, also studied by the writer. 
Incidentally, the cooling of a sill undergoing fractional crystalli- 
zation is seen to be no simple matter. During the formation of a 
solid crystal, latent heat, estimated as about one-fifth of its total 
melting-heat, is given off. The fall of magmatic temperature at 
the level of its growth thus tends to be retarded. The deeper levels 
to which the crystal sinks share practically none of this latent heat 
in a direct way. Apart from other causes, the lower layers of the 
sill magma should therefore freeze first. The level of maximum 
aggregation of sunken crystals cannot be readily foretold, though it 
must be above the quickly chilled floor phase of the intrusive. 
Origin of the pre-Cambrian granites.—Bowen considers all granite 
to be a differentiate of basaltic magma. Most of the continental 
surfaces are apparently underlain by pre-Cambrian complexes, 
chiefly of granitic composition, though so often metamorphosed 
to gneiss. If these stupendous masses of granite represent the 
silicic pole of separation in original basalt, most of the continents 
must now be underlain, still deeper than the granites, by solid rock 
much more femic than basalt and of great aggregate volume. Or 
1Cf. R. A. Daly, Amer. Jour. Sci., XLIII (1917), 423. 
