132 Reviews — Physical Chemistry and Petrology. 



with addition of material. The general effects produced by these 

 agents are discussed in the light of the most recent researches, for 

 many of which the authors themselves are responsible. 



Great emphasis is laid on the distinction between uniform pressure 

 and non-uniform pressure, or 'stress'. It is shown that, for most 

 minerals, under the influence of ' stress' the melting-point is lowered 

 and the solubility increased, but under uniform pressure the melting- 

 point is raised while solubility is not appreciably affected. Further, 

 the condition of uniform pressure is rarely attained in practice, and 

 its effects are much less important than those of ' stress'. 



The general character of the processes brouglit about by the 

 operation of the various factors effective in metamorphism can be 

 predicted, but there is a general lack of accurate quantitative physical 

 data for rock-forming minerals. The utmost caution must be used in 

 attempting to apply to rocks the experimental results obtain(-d for 

 simple systems. Complications frequently arise owing to such factors 

 as the slow rate of reaction, the occurrence of metastable forms or 

 the attainment of positions of false equilibrium. Such complications 

 may have particularly important effects on the application of the 

 phase rule, and especially on the use of transformation points between 

 allotropic modifications of rock-forming minerals as fixed points on 

 a geological scale of temperature. 



2. " The Hydrothermal Formation of Silicates : A Review." By 

 Gr. W. MoREY and P. JSiggli. Journ. Amer. Chem. Soc, vol. xxxv, 

 pp. 1086-1130, 1913. 



Many rock-forming minerals have been synthesized bj' heating 

 their components in the presence of water in closed steel vessels, but 

 such hydrothermal syntheses have rarely been investigated quanti- 

 tatively. The authors have reviewed the work of the earlier 

 experimenters and have compiled a detailed bililiography, giving 

 a brief synopsis of each paper, and a list of minerals obtained by 

 hydrothermal methods. This bibliography is invaluable and should 

 serve as a basis for future research in this field. 



Discussing the theoretical principles of these syntheses, the authors 

 show that all hydrothermal processes are primarily crystallizations 

 from aqueous solutions, and, as such, they may be treated on lines 

 similar to those followed by van't Hoff in his researches on oceanic 

 salt deposits. The relations are necessarily extremely complicated, 

 but it has been found possible to indicate the main points to which 

 attention must be directed in quantitative work. 



3. " The Phenomena of Equilibiia between Silica and the Alkali- 

 Carbonates." Bv P. ]S"iGGLr. Journ. Amer. Chem. Soc, vol. xxxv, 

 pp. 1693-1727, 1913. 



The existence in rock-magmas of volatile constituents and their 

 combination with involatile constituents is well known, but the 

 conditions which control such interactions is little understood. Much 

 light is thrown on this problem by the investigation of the phenomena 

 of equilibrium between alkali-oxides, silica, and carbon dioxide. The 

 system has been studied for temperatures between 900° and 1,000° C. 



