122 REGINALD A. DALY 
micro-pegmatite or myrmekite.* On the other hand, quartz diabase 
is characteristically poorer in alkalies than normal diabase or 
basalt—a relation just the reverse of that expected on Bowen’s 
hypothesis.” 
Hence, neither the mineralogical constitution of quartz diabase 
nor its chemical analyses support the idea that its free quartz is due 
to fractional crystallization, as postulated. 
Gas-controlled differentiation in the liquid phase.—That frac- 
tional crystallization is only one of several important modes of 
differentiation is suggested by the activities of magmatic gases. 
All petrologists recognize the segregation of gas-rich, generally 
salic, material as magmas complete their crystallization. Assuredly 
the gases of mobile fractions have been concentrated by the for- 
mation of gas-poor, solid crystals. However, one cannot assume a 
total absence of gas-rich portions in the original magma nor a total . 
inability of resurgent gases to enter the magmatic chamber. If, 
for these or other reasons, local portions of the magma are or become 
specially gaseous, these would tend to rise in the chamber and so 
bring about differentiation independently of crystallization and, 
it may be, long before crystallization has begun. 
The writer has emphasized the possible derivation of augite 
andesite from basalt by the sinking of crystals. The abundant gas 
which streams through the vents where augite andesite is generated 
‘doubtless lowers the temperature range of consolidation and thereby 
lengthens the time interval during which crystals may settle. It 
may also act as a vehicle for the moderate concentration of silica 
and alkalies in the pipes and thus accelerate the development of a 
magma contrasted with the original basalt.s Lawson expressed 
a somewhat similar idea in discussing the more salic central parts 
of the differentiated diabasic dikes around Rainy Lake.‘ This 
conception does not necessarily imply any degree of immiscibility 
between the gas-rich and the gas-poor phases of the liquid. It is 
tW. H. Collins, Memoir 95, Geol. Surv. Can., 1917, p. 90. 
2See R. A. Daly, Igneous Rocks and Their Origin (New York, 1914), p. 321. 
3 Cf. R. A. Daly, zbid.. p. 377. That anorthositic differentiation takes place under 
contrasted conditions is noted on p. 328 of the same book. 
4A. C. Lawson, Amer. Geol., March, 1891, p. 160. 
