758 REVIEWS 
chapter closes with a brief sketch of various hypotheses that have been 
suggested in explanation of differentiation, and a statement of the processes 
of magma eruption. 
A discussion of structures and the modes of occurrence of extrusive 
and intrusive igneous rocks concludes part one. By the term structure, 
Professor Iddings follows the more recent usage, restricting it to those 
“large features of rock bodies which have been produced by cracking, 
by fracturing and aggregation, or which may be brought about by erosion,” 
the term texture being applied to the microscopic features. 
The second part of the volume, which consists of 121 pages, is taken 
up with nomenclature and classification of igneous rocks. After a short 
historical sketch, the author presents an adaption of the current qualita- 
tive systems of classification. The definitions of the rocks are essentially 
those of Rosenbusch and of Zirkel given in terms of mineral composition 
and texture with no expression of views as to the possible genetic relations 
between them. Professor Iddings divides the rocks into five groups based 
on the dominance of (1) quartz, (2) quartz and feldspar, (3) feldspar, 
(4) feldspar and feldspathoid, (5) feldspathoid. Each of these is separated 
into divisions based on the character of the preponderant feldspathic con- 
stituent, and these, in turn, into divisions with little or much ferromagnesian 
minerals. The final divisions are into phanerocrystalline and aphanitic 
rocks; the latter having two subdivisions, those of cenotypal and those 
of paleotypal habit. 
The last chapter of the book, consisting of 61 pages, is taken up with 
the quantitative classification of Cross, Iddings, Pirsson, and Washington, 
and is an abridgment of the system published by them in 1go02.t The 
rules for the calculation of the norm have been rewritten and are now 
much clearer than formerly, the explanatory notes interpolated in the 
older work being omitted. 
It is only in this second part of Professor Iddings’ book that it differs 
widely from that of Professor Harker. In both books the treatment of 
the newer petrology is similar, but the two writers differ entirely upon the 
question of classifications, although they agree that the existing systems 
are unsystematic, unsatisfactory, and confusing. Professor Harker has 
given but 18 pages to classifications, and one could wish that he had further 
developed his own ideas on the subject, especially in the way of a classifica- 
tion based upon eutectics, such as was suggested by Becker? and Vogt.3 
t Jour. Geol., Vol. X, 1902, pp. 555-690. 
2 Geo. F. Becker, 21st An. U. S. G. S., III, 1899-1900, pp. 519, 520. 
3 J. H. L. Vogt, ‘“‘Ueber anchi-eutektische und anchi-monomineralische Eruptiv- 
gesteine,’’ Norsk Geol. Tidsskr., 1905, Vol. I, No. 1. 
