Reviews — Professor Iddings— Igneous Rocks. 25 



II. — Igneous Rocks. Vol.11. By J. P. Iddings. 8vo; pp. xi, 685. 

 New York : Wiley and Sons, 1913. Price 255. 



WHEN it was announced a few years ago that a group of 

 distinguished American petrologists had decided that the time 

 was come to reform the classification and nomenclature of igneous 

 rocks most students of petrology looked forward with interest to the 

 appearance of a textbook showing how the new classification actually 

 worked out in practice. The most severe and final test of such 

 a proposal would thus be furnished. Professor Iddings' textbook, 

 however, does not meet this demand. It runs very much on the 

 old lines, the 'qualitative' system, so heartily condemned, serving 

 as the basis on which the rock groups are arranged, while the 

 ' quantitative ' classification appears only in short appendices to 

 the chapters, printed in small type, and very interesting, but far 

 from complete. 



This treatment makes the book much more valuable to the student 

 preparing for examination. All the well-known varieties of rocks 

 are described under six divisions according to their component 

 minerals, from those which consist mainly of quartz, to the ultra- 

 basic rocks such as the pyroxenites and peridotites. Each group is 

 subdivided into («) phanerites (coarsely crystalline), and (b) aphanites 

 (finely crystalline or partly glassy). Chemical analyses are given 

 mostly in sepai'ate tables. The new American names appear seldom 

 except in short discussions of the grouped analyses, and the ' mode ' 

 instead of the 'norm' is the dominant factor in the classification. 



One merit of this textbook is that it is well abreast of the times, 

 and the new rock-species recently described from districts such as 

 the Urals, Finland, and Western North America obtain recognition, 

 and at any rate a brief definition. The author occasionally, though 

 all too seldom, gives us a criticism of the validity of the older names 

 such as theralite and essexite (pp. 231 and 247). The first part of 

 this volume, containing the description of the rocks, is only 330 pages, 

 much too short for adequate treatment of the subject, and is hardly 

 sufficient for more than a definition of the established rock-names. 

 It reads very much like a catalogue, and the ' microscopic physio- 

 graphy ' of the different rocks receives little notice. Within its 

 limits, however, the book is excellent, and should be in the hands of 

 all senior students of petrology. 



The second part of the book describes the occurrence or rather the 

 distribution of igneous rocks. We have read it with great interest, as 

 it provides a valuable summary of what is known regarding the 

 igneous rocks of all parts of the world. They are grouped under 

 geographical provinces such as North America, South America, and 

 Europe, and an attempt is made to define also the geological age of 

 the rocks described. Moreover, fairly full references are given to the 

 literature, and a useful set of maps is provided to show where the 

 rocks are situated. Professor Iddings is sceptical about the value of 

 ' petrographical provinces ', and he lays down criteria which, in our 

 opinion, are unnecessarily rigid (p. 346). It is sufficient for the 

 meantime if we can recognize that the rocks of certain districts 

 belonging to a certain epoch have well-marked characters which 



