410 



SCIENCE 



[N. S. Vol. XXX. No. 769 



stances favor differentiation. Instances in na- 

 ture which seem to be explainable by these 

 processes are cited. Contemporaneous veins 

 and pegmatic veins are considered by Iddings 

 as resulting from differentiation and a hypo- 

 thetical explanation for each is given. The 

 facies of composition and texture exhibited 

 by many igneous masses are examined in their 

 relation to differentiation. 



The solution of rock by liquid magma is 

 considered, with the conclusion that evidences 

 of this absorption within the zone available 

 to our examination are rare. Hybrid or mixed 

 rocks are given a brief mention, and a concise 

 historical review of hypotheses of differentia- 

 tion is given. In a short but interesting 

 sketch of the course of magmatic eruption 

 Iddings shows some of the phases or periods 

 which may be supposed to favor magmatic 

 differentiation. 



The last chapter of Part I. is devoted to a 

 description of the modes of occurrence of 

 igneous rocks. Here the course of treatment 

 naturally follows that of other treatises. 

 Numerous excellent and new illustrations add 

 greatly to the attractiveness of this chapter. 



Part II. of the volume deals with " Nomen- 

 clature and Classification," truly a most diffi- 

 cult subject at the present time. On many 

 sides one may hear expressions of extreme dis- 

 satisfaction with the existing condition of 

 systematic petrography and its terminology. 

 But we are in a perfectly natural, though most 

 uncomfortable, stage in the evolution of the 

 science. The man who understands the es- 

 sence of the igneous rock, as presented by 

 Iddings, can best comprehend that nomencla- 

 ture and classification to-day are in confusion 

 because it could not be otherwise. 



Iddings first reviews the growth of the prev- 

 alent nomenclature and of the classifications 

 it expresses. The facts are familiar to petrog- 

 raphers and they are presented by the author 

 in a way to emphasize the conclusion that 

 with such a history the existing confusion is 

 simply the logical result. 



In one chapter Iddings sketches the preva- 

 lent system — if system it can be called — under 

 the term " Qualitative Mineralogical System." 



For the presentation of this system Iddings 

 has recourse to the usual tabular scheme, in- 

 serting the names to be defined in their ap- 

 propriate spaces. Then follows a definition 

 of each name in terms of mineral composition 

 and textura These definitions are essentially 

 as they may be found in the works of Eosen- 

 busch and Zirkel, except the expression of 

 genetic ideas attached by the former. These 

 same names are used with similar significa- 

 tion by German, French, English, American 

 and other petrographers in spite of more or 

 less different bases of classification. 



In the final chapter of the volume Iddings 

 presents a statement of the " Quantitative 

 Classification of Igneous Rocks " of which he 

 is a co-author.' Here is given a criticism of 

 the qualitative mineralogical system and a dis- 

 cussion of the available bases of classification, 

 leading to the choice of chemical composition 

 as the foundation of the quantitative system. 

 This follows closely the original presentation 

 of the system, but is accompanied by many 

 references to facts brought out in earlier parts 

 of the book. 



Taken as a whole this volume leads directly 

 to the quantitative system as the only one yet 

 devised by means of which the petrographer 

 may adequately and correctly express the rela- 

 tions of igneous rocks in regard to their abso- 

 lute, determinable properties as objects. The 

 discussion of the origin of mineral composi- 

 tion and texture is certainly thorough enough 

 to demonstrate the author's deep interest in 

 petrogenesis, but it also serves to show that the 

 complexities, if not the uncertainties, of gen- 

 etic relations render them unavailable as bases 

 of a truly systematic classification of all igne- 

 ous rocks. 



The work is written from a standpoint occu- 

 pied to some extent by other specialists, but 

 which must henceforth be familiar ground to 

 every petrographer worthy of the name. Not 

 that one must agree with Iddings in all re- 

 spects, but that the study and the scientific 

 discussion of igneous rocks must be based on 



' " Quantitative Classification of Igneous Rooks," 

 by Cross, Iddings, Pirsson and Washington, The 

 University of Chicago Press, 1903. 



