560 Revieics — Igneous Rocks. 



is given to the chemical aspects of the subject. The earlier chapters 

 are devoted to a brief account of the chemical characters of magmas 

 and the principal means of representing these by diagrams. AVe then 

 pass to a classification of the elements according to the periodic law, 

 and the rock-minerals are considered according to their dominant 

 components. It is difficult to see that anything is gained by this 

 grouping, and it certainly leads to a good deal of repetition ; the 

 amphiboles, for example, are discussed under calcium, magnesium, 

 iron, aluminium, and sodium. As regards the constitution of the 

 silicates the views of Clarke are followed, but graphic formulae are 

 not used, probably because there is still too much uncertainty in 

 applying them. 



With the third chapter we begin the consideration of the application 

 of the principles of physics and chemistry to magmas, and this portion 

 of the work is entirely satisfactory, both in matter and in style. The 

 exposition is lucid, though the treatment is brief; and for the benefit 

 of those students who have not had a special training in physical 

 chemistry the elementary principles of the subject are carefully 

 expounded. jS^o better introduction to the theory of the crystallization 

 of rock magmas, regarded as solutions, could be provided for the use 

 of students. 



Some years ago a new descriptive terminology for rock structures 

 was proposed by the author and a groiip of American petrographers. 

 Without doubt greater variety and definiteness are needed in the 

 language of descriptive petrology, and many of the terras proposed 

 have already received general acceptance. This nomenclature is used 

 here in the account of the texture of igneous rocks ; much of it is 

 excellent (for example, the suggestion that granophyres should be 

 called graphophyres), and there are other kinds that are by no means 

 unexceptionable. Thus mediophyric and niediiphyric, minophyric 

 and miniphyric, as descriptive of porphyritic structures of different 

 coarseness are more likely to cause confusion than to lead to clearness. 



The book closes with a tabular statement of the ' qualitative ' 

 classification generally recognized in textbooks, and a condensed 

 account of the principles of the new quantitative classification. This, 

 and the chapter on the history of classification, have already appeared 

 in print, and to English readers are likely to prove the least interesting 

 part of the volume, as the space allottecl to either system is too brief 

 for any but a perfunctory treatment. 



One of the most pleasing features of the book is the effort made to 

 avoid the conventional methods of handling the subject, and to place 

 new aspects and new examples before the reader. The author draws 

 largely on the results of field-work among volcanic rocks in Western 

 North America ; and as the petrography of that region has been studied 

 with extraordinary care, and it embraces a remarkable variety of 

 igneous rocks, the instances cited to illustrate the general principles 

 have a freshness and point which impress them on the memory. 



J. S. F. 



