370 PETROLOGICAL ABSTRACTS AND REVIEWS 
study of texture is urged, as it “‘is a very definite exponent of physical 
conditions that attended the crystallization of each igneous magma.”’ 
The accurate determination of the minerals of rocks as to quantity and 
composition is declared essential to a comprehension of the problems of 
igneous rock formation which are specified. The relation of mineral to 
chemical composition and conditions of crystallization is given as one of 
the pressing questions, with two particular phases: (1) the apparent 
absence of molecules called occult which the bulk analysis would indicate 
must be present in certain rocks; (2) the various modes possible in 
magmas of the same composition. The occult molecules are those held 
by recognized minerals in solid solution. Notable illustrations of these 
conditions are given and the difficulty of correlating rocks solely on 
mineral composition is pointed out. 
It is shown that the elucidation of the laws controlling the producwen 
of mineral compounds from molten magmas is a problem for joint attack 
by petrographer, chemist, and geophysicist. 
Under the heading ‘“‘The Mathematics of the Petrology of Igneous 
Rocks” Iddings develops the view that magmas and the mineral aggre- 
gates resulting from their crystallization must be interpreted in terms of 
stoichiometric chemistry and quantitative physics: that igneous rocks 
constitute a series which is unbroken in itself; and that the division of 
such a series into parts for systematic or descriptive purposes is natur- 
ally, because inevitably, by some arbitrary choice of units. The rock 
“type” is declared to be “subjective, inherent in the petrographer, not 
the rock.” 
The abstract quantitative or mathematic relations of the basic 
properties of igneous rocks are dwelt upon to show a fundamental differ- 
ence between them and organisms. Iddings deprecates, therefore, the 
form given the idea of Harker that we may come to a genetic or natural 
classification of igneous rocks based on some “fundamental principle 
analogous with that of descent, which lies at the root of classification 
in the organic world.” The terms ‘“‘consanguinity,” “parent magma,” 
“family,” and others are expressive but should not lead to biological 
analogies, for we are dealing, not with organisms, capable of reproduction, 
but with ‘“‘a series of chemical solutions and their solidified phases,”’ 
although they ‘‘may be related to one another by reason of differential 
diffusion or fractional crystallization. 
Other problems of igneous rocks are discussed by Iddings under the 
head of ‘“Petrographical Provinces.’’ Although himself one of the first 
to recognize and support the generalization outlined by Judd he acknowl- 
