26 H. S. Washington and H. E. Merwin — Note on 



vom Ratk^^ gives the values 2.512 and 2.592 as the 

 densities of the glassy crnsts of Vesnvian bombs of 1872, 

 and Lagorio^^ 2.319 as that of a Vesuvius obsidian. We 

 may therefore provisionally accept a density of about 2.5 

 for the Vesuvius glass. As to the liquid lava, we have no 

 data; but, assuming a density of 2.8 for the solid lava, 

 we may conclude from the discussion of Daly^- based 

 on Mellard Reade's and Barus' data as to expansion, 

 that the molten lava would have a density of about 2.35. 

 This does not take into account the presence of dissolved 

 gases, which would unquestionably lower the density, 

 and at the same time decrease the viscosity, very 

 materially. It is clearly evident, therefore, that augite 

 crystals, placed in such a leucite tephrite magma, would 

 be much more dense than the magma, and would tend to 

 sink, though the actual sinking of many of them would be 

 more or less prevented by movements in the liquid, and by 

 the possible presence of attached gas bubbles in the upper 

 portions of the mass. Anyone who has made mineral 

 separations with hea^y liquids will appreciate the possi- 

 bilities of disturbance of a '^theoretically" perfect 

 separation, adherent particles of the lighter minerals 

 here replacing the gas bubbles of the magma. 



But the occurrence of masses of rock of granitic 

 texture, Avithout euhedral crystals, or crystals formed 

 freely in the body of the liquid, of the same composition 

 as such crystals formed in what must have been a very 

 similar magma and at the same volcano, seems to demand 

 the recognition of some other factor than gravity, or 

 at least one in addition to that of gravity. 



This is not the place to enter into a discussion of the 

 various kinds or causes of differentiation that have been 

 suggested, but I must recall the case of Shonkin Sag and 

 the explanation of its differentiation by fractional 

 crystallization advanced by the late Prof. Pirsson, which, 

 it seems to me, Daly has not adequately met^^ by an 

 appeal to gravitative differentiation. Pirsson and I held 

 much the same views on these matters, and I feel inclined 

 to revive a theory put forward many years ago,^^ chiefly 



^"^ Vom Eath, Z. deutseh geol. Ges., 25, 240, 1873. 



"Lag-orio, Tscherm. Min. petr. Mitth., 8, 475, 1887. 



^^E. A. Daly, this Journal, 15, 277, 1903. 



^' L. V. Pirsson, this Journal, .11, 12, 1901 ; U. S. Geol. Surv., Bull. 237, 

 188, 1905. Cf. Daly, Igneous Eocks and their Origin, 223, 238, 1914. 



"Washington, Bull. Geol. Soc. Amer., 11, 409, 414, 1900; Jour. Geol., 9. 

 663, 1901. 



