654 HENRY S. WASHINGTON 



silica), they will practically disappear if the differentiation con- 

 tinues as indicated by the body of the diagram. Further differ- 

 entiation in this direction then would lead to the production of 

 a purely feldspathic or feldspathoidal rock. If continued still 

 further quartz (free silica) would appear and the rock become 

 aplitic in character. Finally, since the silica is increasing at a 

 rapid geometrical rate, while the other constituents are drop- 

 ping, the extreme result of differentiation in this direction would 

 be pure quartz. 



This inference is obviously in line with the experiments of 

 Barus and Iddings' which indicated that in igneous magmas 

 SiO^ plays the role of electrolytic solvent, analogous to that of 

 HgO in aqueous solutions. This result is also in harmony with 

 the experiments and conclusions of Lagorio^ and Morozewicz,^ 

 who come to the conclusion that the predominant magmatic sol- 

 vent is composed of silica and alkalis, and that it has the power of 

 dissolving large amounts of alumina. The results obtained above 

 would indicate that alumina itself is an essential constituent of 

 the solvent, and it would also seem that there need be no stoichio- 

 metrical ratio between the four constituents. As has been 

 indicated above, however, it will not do to push conclusions too 

 far from such meager data, and it is by no means necessary to 

 infer that a rock solvent of this character is the only possible 

 one. But it will be as well to defer all discussion of these topics 

 until more complete data are available. 



The comparative rarity of occurrences of purely or very 

 highly siliceous igneous rocks may presumably be ascribed to 

 the fact that long before this phase of differentiation has been 

 reached, the mass will, in most cases, have become solid (owing 

 to the high melting point and great viscosity of such mixtures), 

 and hence incapable of further change in this way. 



It is of interest to note in this connection that a specimen 

 of "aplite" has been collected by Dr. Weed, and is now in the 



'Barus and Iddings, Am. Jour. Sci., Vol. XLIV, p. 248, 1892. 

 ^Lagorio, Mi7Z. Pet. Mitth., Vol. VIII, p. 508, 1887. 

 3MOROZEWICZ, Min. Pet. Mitth., Vol. XVIII, p. 235, 1899. 



