Geology. 665 



Wulffs' treatise on volcanism. But a systematic gathering and 

 presentation in well-digested form of the great mass of material 

 which in recent years has accumulated in the domain of physical 

 chemistry and has a bearing on petrogenesis, is much to be 

 desired. In this country the work of Day and his co-laborers in 

 the Carnegie Geophysical Laboratory is well known ; that of 

 other investigators in other places, less directly connected with 

 geological problems, is often quite unknown to penologists. 

 Moreover, the relative value of much of the material, and its 

 bearing on the problems of petrology, are not directly evident. 

 Yet it is certain, as the author remarks, that it is along this induc- 

 tive path of experimental synthetic investigation that the greatest 

 progress in the science, in its present stage of development, is to 

 be made. From what has been said the general nature of the 

 work is evident, but the following details will give an idea of its 

 plan and scope. After an introductory statement respecting 

 homogeneous and heterogeneous equilibrium, the author takes up 

 the general subject of magmatic petrogenesis, which comprises 

 about one-half of the volume. Under this heading he discusses a 

 variety of topics, such as the melting point of minerals under 

 varied conditions ; the properties of siliceous melts, such as vis- 

 cosity, surface tension, electrical conductivity, density, etc.; 

 enantiotropism and monotropism, the thermal properties of min- 

 erals, etc.; and then treats first of two component systems, under 

 which eutectics, isomorphism, and solid solution are comprised; 

 and afterwards of systems of three, four and more components. 

 Finally a review of the chemico-pbysical, and especially the 

 thermal properties of the main rock-making minerals, individually 

 considered, closes this portion of the work. 



In the second part the volatile constituents of magmas are 

 treated; their properties, solubility in molten solutions, the equi- 

 librium of gases, etc., etc. 



The author then, in the third part of the work, discusses the 

 pegmatitic, pyroaqueous and hydrothermal phases of magma 

 solidification. Here such subjects as the properties of water at 

 high temperatures, critical temperatures and pressures, the forma- 

 tion of minerals in systems with volatile components, and a 

 variety of other matters which have important bearings on the 

 formations of certain rocks and ore-deposits, are treated. 



Next comes the subject of weathering, then of sediments, under 

 which latter the problems of dolomite and salt deposits are con- 

 sidered, and the work concludes with a section on metamorphism. 



This sketch will give the reader a notion of the ground cov- 

 ered. While such a work is necessarily one of compilation, and 

 the writer has selected his material well and expressed it concisely 

 in clear language, it is noticeably impersonal in tone, and edi- 

 torial comment and criticism are on the whole rare. Where such 

 occurs they are quite conservative in character. It is rather in tha 

 selection of the material that the author passes judgment. The 

 work has, therefore, a rather synoptic and somewhat statistical 



