GEOPHYSICAL LABORATORY. 171 



(28) Pressure as a factor in the formation of rocks and minerals. John Johnston. J. Geol., 



23, 730-747 (1915). 



This is a brief general discussion of the available experimental evidence 

 bearing upon the influence of pressure on melting and transformation points 

 and solubility, and of the inferences with respect to the formation of rocks 

 and minerals which may justifiably be drawn from this evidence. In general 

 it would seem that the importance to geology of the effects of pressure upon so- 

 called physical changes (e. g., the melting-point of a single pure substance) has 

 been overestimated relatively to that of the influence of pressure upon chem- 

 ical changes — in other words, upon equilibrium in polycomponent systems. 

 Change of effective pressure will in general change the configuration of the 

 various fields of stability in a system, acting thus in a way precisely analogous 

 to change of temperature, or of gross composition ; but in the case of pressure 

 the effect will usually not be especially marked unless one or more of the 

 components is volatile; that is, unless the concentration of one or more of the 

 components really changes appreciably with change of pressure. This is 

 merel}^ an example of the general rule that the magnitude of the effect of pres- 

 sure on a system depends upon the difference in compressibility of the several 

 phases present, being greatest when this difference is greatest, and conversely. 



Accordingly we must, in any discussion of the course of crystallization from 

 a complex magmatic system, take into account the mode in which the effective 

 pressure varies as well as the mode of cooling. For change of pressure, like 

 change of temperature, may affect the order of crystallization, and even the 

 character of the minerals which separate, this result of course depending 

 merely upon the circumstance that the saturation limits (solubilities) of the 

 several solid phases which could possibly separate out are not all affected 

 equally by change of conditions. 



(29) Druck als ein Faktor der Mineral- und Gesteinsbildung. John Johnston. Neues 



Jahr. Min. Geol., ii, 89-108, 1915. 



A German translation of "Pressure as a factor in the formation of rocks 

 and minerals" (J. Geol., in press). Reviewed under No. 28 above. 



(30) The sulphides of copper. Eugen Posnjak and E. T. Allen (thermal and chemical 



study); H. E. Merwin (microscopic study). Econ. Geol., 10, 491-535. 



Cuprous sulphide was prepared in a vacuum furnace and its melting-point 

 determined therein; the latter was found to be 1130°±1°. No dissociation 

 of cuprous sulphide takes place at 1200°. The specific gravity of synthetic 



cuprous sulphide, — js — , was found to be 5.785; practically the same 



as the specific gravity of the purest natural mineral analyzed, viz, 5.783, 

 mineral 25° 



water 4° 



The copper sulphides obtained by fusing together copper and sulphur are 

 of variable composition and always contain more sulphur than the ratio 

 2Cu : S. They are microscopically homogeneous and vary continuously both 

 in color and in specific gravity with composition; as the sulphur increases 

 they become darker in color and lighter in gravity. The specific volumes of 

 these products lie within the Umits of error on the straight line connecting the 

 specific volumes of cuprous and cupric sulphide. It has been shown that these 

 products are solid solutions of cuprous and cupric sulphide. 



Copper sulphide melts in an atmosphere of hydrogen sulphide at 1096°; 

 in an atmosphere of sulphur vapor at 1057°. The lower melting-point as 

 compared with that of cuprous sulphide is due to dissolved cupric sulphide, 

 which increases with the increase of the sulphur-vapor pressure surrounding it. 



