126 REGINALD A. DALY 
reasons for assuming assimilation on an appreciable scale “‘are 
considered to be entirely removed when it is shown that the types 
enumerated above and probably all other igneous rocks could be 
derived from basaltic magma by differentiation alone.” A lapse 
of logic is indicated by the italicized words. 
Hybrid rock not the normal result of assimilation.—One must 
doubt that the ‘‘formation of an obviously hybrid rock” should be 
the “normal result of assimilation” (Bowen, p. 85). A fused 
xenolith or the solute slowly diffusing away from its source into 
the dissolving magma, tends, as already noted, to rise (or sink) in 
the general body of liquid. As the foreign material moves under 
gravity it is more and more mixed with the original magma. This 
more dilute solution, retaining nearly all of its original temperature, 
is about as likely to separate as any part of the original magma itself. 
The special composition of the absorbed rock may of itself stimulate . 
strong differentiation. Under the conditions hybridism is far from 
being obvious. 
The lowering of crystallization temperature by mixture may 
have little to do with the differentiation of many syntectics. Back 
of all theorizing are two fundamental questions: How greatly is 
primary, dissolving magma superheated ? What is the ratio of its 
volume to that of its solute? In other words, how much work is 
supposed to have been done and what was the amount of energy 
available ? 
The petrologist should think always to scale, in space! 
Meaning of chilled contacts in batholiths —Bowen states (p. 84) 
that little assimilation can be admitted in the case where a batholith 
has a chilled contact against its country rock. Ii stoping estab- 
lished the visible contact, the magma was nearly frozen; for, if it 
had been very hot, further stoping would have occurred. The 
visible contact was made after the sinking away of the last batch of 
shattered blocks from the main country rock. The chilling effect 
is thus due to the relatively low temperature of the new, last surface 
of contact made with the invaded formation. In that case the 
chilling phenomena give no trustworthy indication of the assimilat- 
ing power of the batholith during its enormously long magmatic life. 
The petrologist should think always to scale, in time! 
