178 Revieivs — Hints on Collecting. 



As regards tte rest of the book, it is practically a reprint, with a few 

 additions and corrections, of the fifth edition (1909) of the " Text-book 

 of Petrology ". The fact that this book has run through so many 

 editions is suflS.cient proof of its excellence, and it is so well known 

 that it is quite unnecessary to call attention to its merits. The additions 

 are very few : the subject of dynamic metamorphism is treated rather 

 more fully, and a table of mean refractive indices of minerals has 

 been added. Birefi'ingence is still stated as ' weak ', ' moderate ', or 

 ' strong ', though it has been pointed out that to give the actual 

 values of the greatest and least refractive indices would be the more 

 satisfactory procedure, and would make this part of the book far more 

 useful. 



The classification of igneous rocks which was introduced in 1909 

 has been retained and, in the case of the gabbros, a further subdivision 

 into a labradorite and an anorthite series has been suggested. Objection 

 has been raised to Dr. Hatch's classification on the ground that 

 it is neither consistently mineralogical nor consistently chemical. 

 However, no classification is free from such objections, and Dr. Hatch's 

 scheme is one which has been found to have many advantages. 



In revising the descriptive part of the book the author has not 

 everywhere succeeded in including the results of most recent work — 

 in fact, there are very few references to any work published since 

 1910. Thus borolanite is still classed with the alkali-gabbros, though 

 in a recent paper Dr. Shand has pointed out its affinities with the 

 syenites and has described new rock-types associated with it. 

 ' Spilosite ' on p. 29 is obviously a misprint for ' spilite ', but this is the 

 only mention made of this rather important group of rocks, and no 

 reference is made to the important paper by Dewey & Flett, published 

 in this journal in 1911. 



The printing is done very well, and the book is vastly improved by 

 the reproductions of a large number of excellent photographs by the 

 Geological Survey and by Dr. H. H. Thomas. 



III. — Hints on Collecting Geological Infoemation and Specimens. 



By G. W. Grabham and Stanley C. Dunn. pp. 24. Khartoum : 



published by the Sudan Government. Price 6^. 

 rilHIS little book is the work of the two geologists who constitute 

 J_ the personnel of the Geological Survey of the Anglo- Egyptian 

 Sudan. "When the task of surveying an area of nearly a million 

 square miles is allotted to so small a staff of professional geologists 

 progress must necessarily be slow, and it has been thought desirable to 

 enlist the services of other Government officials to furnish the Survey 

 with geological information concerning the districts its members are 

 not able to visit. These directions have accordingly been issued 

 to afford some guidance as to the information to be furnished and the 

 specimens to be collected. The latter subject might perhaps have 

 been more fully treated, for a good collection representing the rocks, 

 minerals, and fossils of an area is more likely to be of real value than 

 attempts at geological description by those who have an imperfect 

 knowledge of geological principles and little or no experience in 

 field work. Owing to the circumstances of the country special 



