282 PETROLOGICAL ABSTRACTS AND REVIEWS 



Methods of specific gravity determinations and of separation of rock 

 constituents are given, including a description of each of the various 

 heavy solutions and melts used for such purposes. Microchemical tests 

 are briefly presented, the treatment being limited to special methods for 

 distinguishing certain particular minerals and groups. 



Methods for the study of opaque minerals are but briefly referred to. 

 Considering the growing importance of this part of the subject, it would 

 seem desirable that this section should be expanded, as the author him- 

 self remarked in his preface. 



The book ends with a discussion of the preparation of thin sections, 

 collecting, filing, and cataloguing of rock specimens and sections, and an 

 appendix of useful mathematical formulas, recipes, etc. 



A special feature of the book is the references to the original litera- 

 ture, which are representative and comprehensive even in those parts of 

 the subject which are but briefly treated in the text. 



The illustrations are abundant and excellent. Special mention may 

 be made of the beautiful half-tones (after Hauswaldt) of interference 

 figures. And in this connection it may be noted that much credit is due 

 the publishers for the excellent book work — a feature which adds much 

 to the readability and usefulness of the book. 



The multiplicity of methods described would probably be confusing 

 to most beginning students, yet the descriptions are presented as far 

 as possible in a simple and elementary way, taking little for granted, and 

 thus by a judicious selection of the articles to be studied, the book could 

 be made use of in an elementary course. Numerous cross-references are 

 helpful in making rapid reference possible to related or explanatory 

 sections. 



For a first edition of a book of this character, dealing with numerous 

 details of description, tables, and formulas, it appears to be remarkably 

 free from infelicities or errors of detail in statement or print. With no 

 attempt at completeness, a few may be noted. 



P. 20, throughout discussion, a should read c to agree with Fig. 35. 



P. 33, article 25, Une 10, "If a is constant," read w for a. 



P. 67, the row of figures 113 to 118 should be inverted, the figure at 

 the right being 113. 



It would seem desirable to use some such expression as ''normal 

 velocity surface" instead of "wave surface" in the discussions of pp. 

 75 flf., and also pp. 97 and 98. Light passing from the center point may 

 be ideally conceived as actually passing out in the form of the so-called 

 ray surface, but it cannot be imagined as spreading out in the form of 



