400 N. L. BOWEN 



pressures. The suggestion is that high pressure may induce under- 

 cooHng, and that immiscibihty may result from the undercooHng. 

 This suggestion arose from experiments made by Bridgman, in 

 which undercooling of liquids became more marked under higher 

 pressures. Daly imagines that magmas may become similarly 

 undercooled under pressure, and that in the undercooled condition 

 unmixing may occur. Assuming the undercooling, it must be 

 admitted that a certain possibility of the formation of non-consolute 

 liquids is thereby introduced, for examples are known in which 

 mixed liquids that are otherwise completely miscible develop 

 immiscibility when undercooled. But such liquids reveal this 

 peculiarity by having a freezing-point curve of a peculiar and char- 

 acteristic shape. ^ Curves of this shape, or even remotely suggesting 

 it, have not been noted among the liquidus curves for silicates, so 

 that it may be regarded as unlikely that undercooling can develop 

 immiscibility in silicates. 



Quite apart, however, from the question of the probable effect 

 of undercooling, we may point to the complete failure of igneous 

 rocks to show evidence of undercooling when occurring in the largest 

 masses available for human inspection, viz., batholiths. Indeed, 

 if we consider the series that begins with a glassy lava, passes 

 through a glassy lava with spherulites, through felsitic and fine 

 granular textures, and ends with a coarse granular panidiomorphic 

 texture, we find definite evidence of a continuous decrease in the 

 degree of undercooling under which crystallization took place. 

 The abyssal rocks are therefore characterized by a decidedly 

 limited degree of undercooling, and the great magma reservoirs (if 

 by this is meant something deeper than batholithic masses that 

 become exposed) are exceedingly unlikely seats of significant 

 undercooling. All of this is in accord with the laboratory experi- 

 ence that if one wishes to undercool a substance he must use a 

 small quantity of it. The particular conditions not covered experi- 

 mentally and urged by Daly as possible conditions favoring immisci- 

 bility do not, therefore, seem promising in that direction. 



The writer's second reason for rejecting immiscibility is not 

 adequately stated by Daly. He states it as being the "high 



'Boeke, Grundlagen der physikalisch-chemischen-Petrographie, p. 113. 



