THE GENESIS OF IGNEOUS ROCKS 319 



a real significance from the petrogenetic standpoint." " Struc- 

 tures which depend on the special conditions under which a 

 magma consolidates, and are not directly related to its compo- 

 sition and origin, are clearly of a lower order of importance." 



There is no space to do justice to the earlier chapters 

 dealing with the geological relations of igneous rocks, a 

 subject which the author has made peculiarly his own. He 

 lays great stress here, as in some of his previous writings, on 

 the importance and widespread occurrence of the succession : 

 volcanic rocks, plutonic rocks and minor intrusions ; though he 

 would not deny that some intrusive rocks must be connected 

 with contemporaneous volcanic phenomena. We are indebted 

 to him for the introduction of the term "minor intrusions" 

 (also employed in his memoir on the igneous rocks of Skye) 

 for the so-called "hypabyssal" rocks, which are often no less 

 deep-seated than the plutonic or " abyssal " rocks. These latter 

 might, by an extension of the same nomenclature, be more 

 happily referred to as " major intrusions." 



The book can be confidently recommended to all advanced 

 students of petrology. It furnishes abundant material for 

 thought, and indicates the directions that research must take in 

 the immediate future. Its appearance should be followed by a 

 striking development in this country of original work on the 

 genesis and relations of igneous rocks. 



Almost simultaneously another treatise dealing with the 

 general principles of petrology has been published in New York 

 by one who has for many years been identified with petrological 

 research in the United States. While Mr. Harker set himself 

 to discuss the more important recent advances and the problems 

 awaiting solution, Professor Iddings has in this, the first volume 

 of his Igneous Rocks, given us a comprehensive text-book on 

 the principles of the subject. It has a wider scope, and subjects 

 dealt with in both treatises are necessarily treated more briefly. 



Considerable attention is given to chemical data and prin- 

 ciples connected with magmas and igneous rocks. A number 

 of typical analyses are furnished, and the different forms of 

 diagrams employed to express the chemical composition of rocks 

 are explained and illustrated in some detail. There is a useful 

 discussion on the part played by the different elements in 

 igneous magmas and in the minerals that crystallise out from 

 them. An interesting suggestion is made to explain why 



