152 Scientific Intelligence. 



The second objection that will be advanced is the difficulty of 

 telling in a great number of cases to which one of the three 

 zones, or orders, the rock of a certain group belongs to. This is 

 only another instance of the general difficulty of attempting to 

 draw arbitrary lines between rocks, which grade into one another 

 in various directions, and only time and experience can show 

 whether it is too great, in the way proposed, to cause its general 

 acceptance. The author modestly states that his work is not 

 considered final, but as an attempt to introduce a rational and 

 definite method of classification, based on consistent principles, 

 into the crystalline schists. For this he is certainly to be com- 

 mended and his work should be carefully read and considered by 

 every petrographer. l. v. p. 



13. Rock Minerals: Their Chemical and Physical Characters 

 and their Determination in Thin Sections ; by Joseph P. 

 Iddings. Pp. xii, 548. New York, 1906 (John Wiley & Sons). 

 — It is now nearly twenty years since the volume of Rosenbusch 

 on the characters of minerals important in petrography was pre- 

 sented in English form by Professor Iddings. This translation 

 has served a most useful purpose in making the work of Rosen- 

 busch accessible to English readers, and has done much to stimu- 

 late study in this department. Since that time, the original 

 work has been revised and enlarged until now its scope is so 

 extensive as to put it beyond the use of the ordinary student. 

 Professor Iddings has done well, therefore, in preparing an inde- 

 pendent work of his own on the subject, which while giving all 

 the data required, presents them in a form which is most con- 

 venient for the special object in view. The result of his labors 

 is now given to the public in this volume of 550 pages, divided 

 almost equally between the general theoretic portion and the 

 description of species. 



Part I begins with the chemical characters, discussing briefly 

 the relation of mineral compounds on this side, and giving some- 

 what more fully the special chemical tests which can be applied 

 to the study of rocks in thin sections. Then follows a chapter 

 upon the general physical properties and crystallography treated 

 quite concisely. The third chapter, occupying about one hun- 

 dred pages of the work, is given to the optical properties. This 

 the author has elaborated with a great deal of care and with the 

 result of giving an admirable summary for the use of the student. 

 He starts with the fundamental conceptions of wave motion, and 

 the simpler light phenomena, and develops the properties of 

 doubly refracting crystals very clearly and in all necessary detail. 

 This portion of the work is particularly valuable to those studying 

 rocks under the microscope, since the phenomena involved in the 

 detei'mination of species are more or less intricate, and unless the 

 principles upon which they are based are thoroughly understood, 

 the results reached are not likely to be of great value. The rock 

 minerals are taken up in succession, commencing with the feld- 

 spars, and are treated as fully as is necessary for the special 



