4i8 REGINALD A. DALY 



peridotites have been formed through gravity separation of basic 

 minerals; such a view is manifestly wrong. 



6. The rival hypothesis of magmatic differentiation. — Finally, the 

 hypothesis should meet the test of showing superiority over an 

 obvious alternative suggestion. It might be conceived that augite 

 andesite is due to the splitting of the basaltic magma before crystal- 

 lization set in. In this view one must suppose that the splitting is due 

 to a drastic change of physical conditions when the lava passes from 

 abyssal levels to levels at or near the surface. The pressure is cer- 

 tainly very different in the two positions. However, no facts are in 

 hand to show that a release of pressure causes immiscibility between 

 two such magmas as molten andesite and molten peridotite, the two 

 actually observed differentiates. If release of pressure alone caused 

 the spontaneous splitting of olivine basalt into lighter andesite and 

 heavier peridotite, we should very rarely expect to find true olivine 

 basalt on the earth's surface, for most of these fluid lavas spend a 

 considerable time in their conduits before being erupted. Against 

 the idea is, further, the absence of any known physical reason why 

 release of pressure should cause immiscibility. Experiments seem, 

 on the contrary, to show that increase of pressure tends to promote 

 immiscibility. 



Immiscibility might conceivably ensue through a fall of tempera- 

 ture from a superheated condition, but there is no direct evidence that 

 the phenomenon has a place in the history of volcanic cones or fissure- 

 vents. 



However probable may be the doctrine of immiscibility for certain 

 magmas under plutonic conditions, we may regard the evidence on 

 the problem as negative so far as volcanoes are concerned. The case 

 may be summed up thus: The basaltic magma may split spontane- 

 ously into two or more magmas at a volcanic vent, but the phenocrysts 

 of a molten basalt must sink while the lava undergoes the extremely 

 slow cooling within the vent. Possibly both methods of differentiation 

 are active. For the one advocated specially in this paper the physical 

 conditions are quite simple and are largely understood quantitatively. 

 The alternative view is at present a somewhat elusive conception for 

 the petrologist and has few field-observations or compelling deductive 

 considerations in its favor. It should, however, be added that the 



