372 PETROLOGICAL ABSTRACTS AND REVIEWS 
by the belief that genetic relations and some of the characters of igneous 
rocks are not adapted to serve as bases of their systematic petrographic 
classification, i.e., the one system on which their scientific nomenclature 
rests. In this paper the availability of various factors is given renewed 
discussion. Under the heading “Factors of Chemical Composition,” 
objection is made to the primary distinction of Atlantic and Pacific 
Kindred or the alkaline and calcic branches as inaccurate and vague. 
It is pointed out that if the differences of composition are partly due, 
respectively, to primeval variation, magmatic differentiation, and 
assimilation, three independent causes, the laws of differentiation alone 
cannot be applied to all rocks in a systematic way. 
In considering the propositions emanating from Rosenbusch to use 
these laws in formulating the current system the distinction of two great 
magmatic series and the division of ‘‘dike rocks” are characterized as 
really unnatural. Instances of “dike rock”’ association and occurrence 
conflicting with the basal generalization are given. 
Classification by eutectics is reviewed, with reference to the proposi- 
tions of Becker and Vogt, and rejected, as inapplicable to all rocks, and 
based on a part of the rock at best. It is furthermore extremely hypo- 
thetical at the present time. 
Factors of mineral and textural characters are next considered and 
the well-known fundamental objections to their systematic use are 
reviewed. The former is not only too complex for practical use but we 
now know that it is not, as once assumed, a simple function of chemical 
composition, but depends partly on variable and independent physical 
conditions. 
The section of this paper dealing with the quantitative system is 
devoted to correcting certain misconceptions, to explaining that the norm 
is largely a means of expressing the molecules actually in magmatic 
solution and not a pure figment of the imagination, and to reviewing 
certain criticisms, particularly some expressed by Harker. 
WHITMAN CROSS 
