Reviews. 



Sofne Queries on Rock Differe7itiatio7i. By G. F. Becker. Amer. 

 Jour. Sci. (4), Vol. Ill, pp. 21-40, January 1897. 



In this discussion Dr. Becker views his subjects from the physico- 

 chemical side. He attempts to apply some of the results of Van 

 t'Hoff, Oswald, Nernst, and other investigators to the question, "How 

 may rock segregate into portions of distinctly different, yet of allied 

 composition ?" He thinks that differentiation, or segregation, as he 

 prefers to call it, can occur only in two ways: (1) By the increasing 

 or decreasing the concentration of certain components, caused by 

 variations in temperature or pressure, which exist in different portions 

 of the magma. These components are thought to be dissolved in the 

 remainder of the liquid rock, and to obey the laws of dilute solutions. 

 (2) By separation into two immiscible liquids due to changes in tem- 

 perature and pressure. These take place by means of molecular flow, 

 or diffusion, and he thinks convection would hinder this segregative 

 process. He gives as instances of molecular flow the diffusion of salts 

 in water, and explains the relation of osmotic pressure and diffusion 

 to gas pressure and diffusion, and mentions that the three funda- 

 mental laws of gases, viz., Boyle's, Gay-Lussac's and Avagadro's have 

 their parallels in three similar laws of solution. 



Two cases of diffusion are next considered: (i) The diffusion 

 caused by heating a solution at the top. (If the solution is heated 

 at the bottom, convection commences and segregation is prevented.) 

 (2) By a difference of pressure at the bottom from that at the top. 

 Any diffusion caused by differential pressure would be too slight to 

 have any appreciable effect. As to (i) it will appear from arguments 

 yet to be presented that the diffusion will be so slow that in entire geo- 

 logical periods no effective change of composition would be possible. 

 In his paragraph on the character of diffusion he states the units by 

 which diffusion is measured, and that the rate of diffusion is expressed 

 by the same mathematical formula as is the conduction of heat. 



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