Reviews — Physical Chemistry and Petrology. 131 



according to the bases, and not by their acid radicles ; elements are 

 treated in the chemical groups in wliich they fall. Thus in group 

 viii come minerals containing iron, nickel, and cobalt; rutile, zircon, 

 and cassiterite fall together in group iv, sodium and potassium 

 compounds in group i, and so on. The groups are taken in inverse 

 order, commencing with group viii, and the silicates are taken last as 

 a special division. 



In the treatment of the silicates the optical properties and means 

 for determining the minerals in thin sections are described. 



An account of the use of Szabo's flame-reactions for tlie determina- 

 tion of felspars, with simple apparatus for its application, is very 

 welcome. It is in this part of the book that the author has shown 

 himself fully alive to the points which are most likely to perplex the 

 student. In describing the measurement of the extinction directions 

 in augite a notable point is made. In most textbook diagrams 

 showing the extinction directions of augite in the plane of symmetry 

 the figure is bounded by the faces (100) and (001), whereas in rock- 

 sections the bounding edges are usually the traces of (110) and (1 11) ; 

 thus the extinctions must be measured towards the upper acute angle 

 in rock-sections, but towards the upper obtuse angle of the diagrams. 

 It is unfortunate that a misprint has crept in here ; the face given as 

 (111) on p. 305 should obviously be (111). 



The book is full of references to recent mineralogical work, and is 

 thoroughly up to date. The student of geology who works through 

 this book should be able to turn to petrology with a real knowledge 

 of minerals and their optical properties, and should be in a position 

 to grasp the reasons for the phenomena which he observes in thin 

 sections. It is for this reason that the silicates have been reserved to 

 the end. To quote Professor Cole : " The geologist . . . reaches the 

 silicates as a sort of climax. His most difficult work comes last, and 

 the fact that by this time he is familiar with a variety of minerals 

 that he has handled, and with specimens where crystalline form 

 is readily apparent, will prevent him from regarding the earth as 

 composed of bodies presented to his eye as thin sections under the 

 microscope." \y_ q g_ 



V. — Physical Chemistry and Petrology. 



Notes on some recent work at the Geophysical Laboratory of 

 THE Carnegie Institdtion of Washington. 



1. " The General Principles underlying Metamorphic Processes." 

 Bv J. Johnston and P. Niggli. Journal of Geology, Chicago, 

 vol. xxi, pp. 481-516, 588-624, 1913. 



THIS paper discusses the validity, limitations, and relative 

 importance of such physico-chemical principles as are operative in 

 metamorphic processes. The term raetamorphisni is used in a restricted 

 sense, being limited to those cases in which "the effects produced by 

 the alterations determine completely the character of the rock mass". 

 The agents concerned in such cases include temperature, uniform and 

 non-uniform pressure, mass-action, the phase rule, and raetamorphisni 



