_ >sv R. A. Duly — Mechanics of Igneous Intrusion. 



The solubility of rock-forming minerals in silicate magmas 

 has been shown by fusion experiments to depend on a, the 

 temperature of the magma ; b, the chemical composition and 

 fluidity of the magma; c, the fusibility of the minerals; and, 

 d, on pressure. Doelter has been able to prove that, under 

 one atmosphere of pressure, all the common types of rock- 

 forming minerals are completely soluble in certain representa- 

 tive magmas at temperatures only slightly above those of their 

 respective consolidation points. These magmas were made 

 from granite, obsidian, common basalt, limburgite, phonolite, 

 foyaite, leucite basalt, leucitite, hornblende andesite, and 

 nepheline basalt — a magmatic range so wide as to demonstrate 

 the practical certainty that all silicate magmas have similar 

 solvent properties. He further shows that the melting point 

 of a silicate rock occurs at about the average temperature of 

 fusibility of its constituent minerals. Long before, Bischof 

 easily dissolved clay-slate in fluid lava, using a bellows furnace 

 for fusion.* These important deductions from laboratory 

 investigations correspond to the facts of outdoor nature. Well- 

 known practical examples may be found in the fused and 

 greatly corroded granite inclusions in the basalts of the 

 Auvergne ; and, again, in the complete disappearance, by fusion, 

 of the " floating islands " in the caldera of Kilauea.f The 

 high fluidity of the normal plutonic magma would likewise 

 facilitate the complete solution of foreign fragments, as experi- 

 mentally proved by Doelter. 



It is true that the direct influence of pressure tends to 

 elevate the melting points of silicate mixtures, though 

 probably not in a degree proportional to the amount of the 

 pressure.;); Yet that effect on the solvent power of the magma 

 may be much more than counterbalanced by the indirect effect 

 of pressure in retaining water and other solvents. Once molten, 

 pressure tends to keep silicate magmas molten, since it lowers 

 the temperature point of consolidation^ In determining the 

 solvent power of a plutonic magma, temperature furnishes 

 here, as in fixing the melting-point, the " coarse adjustment," 

 as pressure furnishes, of itself, the "fine adjustment." 



In conclusion, then, it seems legitimate to regard the condi- 

 tions of the abyssal portions of plutonic magmas as conspiring 

 toward the perfect digestion of a submerged foreign rock frag- 

 ment during all the time of intrusion except during the short 

 period preceding final consolidation. Even so uncompromis- 

 ing an opponent of the theory of contact digestion by stock 

 magmas as Brogger admits that such assimilation can be, in 



*Chem. u. Phys. Geol., Supplement, p. 98 (1871). 



f J. D. Dana, Characteristics of Volcanoes, p, 176, New York, 1891. 



j Doelter, Tscher. Min. u. Petrog. Mitth., xxi, 221 (1902). 



§ Oetling, op. cit., p. 370. 



