PETROLOGICAL ABSTRACTS AND REVIEWS 487 



4(K20.A1A)- eCNa^O.ALOj). 2(Na20. FeA)- These rocks may contain free 



quartz (or nephelite) . Where there is quartz it is to be noted that only four 



molecules of Si02 are required to one of NaaO, consequently there will be more 



quartz than in the case where all the soda goes into feldspar. The formula for 



the quartz, in this case, consequently, reads 2[(Si02mol. — 50) — S 2(K20 mol.-j- 



NazO mol.)+aeg. mol}]. 



With insufficient silica, so that a negative result is obtained by the quartz 



formula, it is evident that olivine or nephehte (or leucite) is present. The 



formula here is divided instead of multiplied by 2, and we have nephelite 



Si02-5o-[2(K20+Na2O)+aeg. mol. ] , , „ ,. , , ,,, • . 



= (the alkali molecules are those m 



2 



feldspar and feldspathoid only). 



Examples of the nephelite rocks and rocks with insufficient alumina would 

 make this abstract too long. 



The third section of the book consists of a natural classification of rocks on 

 genetic principles. A detailed description is impossible here and the reader is 

 referred to the original work which is full of suggestions. Briefly, rocks are 

 divided into five groups: (i) the orthogene class of rocks includes those 

 formed by slow cooling in the interior of the earth; (2) the paragene class 

 originated in a relatively rapid cooling of the magma during the period of 

 crystallization of olivine, consequently that mineral could not settle out but 

 is represented by its recrystallizations as diopside, etc.; (3) the hypogene 

 class includes extrusive rocks formed under conditions of rapid cooling, relief 

 from pressure, and movement; (4) peratogene rocks are those altered by 

 metamorphism and include the crystalline schists; (5) diagene rocks include 

 the sediments. The first four groups are subdivided into ten zones each, each 

 zone representing a distinct temperature-interval through which the magma 

 passed on cooling, and marked by the separation of typical and chemically well- 

 defined minerals. There are thus the zones of (i) chromite, (2) ohvine, (3) 

 enstatite, (4) diopside, (5a) labradorite, (sb) nephelite, (6) labradorite-ande- 

 sine, (7) andesine, (8a) andesine-oligoclase, (8b) oligoclase, (9) orthoclase, 

 (10) quartz. 



The book is one which will repay very careful study in every part. While 

 one might wish that the "Konstitutionsformel" represented actual rather than 

 theoretical minerals, one can easily understand how the former might make 

 difficulties in certain cases. Undoubtedly this book is one of the most impor- 

 tant works on petrography that has appeared in recent years. 



Iddings, J. p., and Morley, E. W. "A Contribution to the Petrog- 

 raphy of the South Sea Islands," Proc. Nat. Acad. Set., IV 

 (1918), 110-17. 

 This is a preliminary statement of the geological structure and character 

 of seven islands of the South Seas, including Tahiti, Moorea, and the Society 

 Group. Thirty new chemical analyses are here published for the first time. 



