378 Reports and Proceedings—Geological Society of London. 
REPORTS AND PROCHEDIN GS. 
I.—Gerotogicat Socrrery or Lonpon. 
June 15, 1910.—Professor W. W. Watts, Sc.D., M.Sc., F.R.S., 
President, in the Chair. 
The following communications were read :— 
1. ‘The Natural Classification of Igneous Rocks.” By Dr. Whitman 
Cross, F.G.S. 
The author reviews the various sytems of classification which have 
been proposed. He discusses the origin of the difference of composition 
of igneous rocks due to: (1) primeval difference, (2) magmatic 
differentiation, (3) assimilation; and points out that differentiation 
and assimilation are in a measure antithetical processes. 
If the deep-seated magmas of large volume have acquired their 
various chemical characters in different ways, it appears at once 
evident that this primary genetic factor cannot be used in classi- 
fication, unless the characters of different origin can be distinguished 
in the rocks. 
Classification by geographical distribution of chemically different 
rocks is considered, and the groupings proposed by various writers 
are discussed ; and it is shown that the rocks of the Pacific zone of 
North America indicate that they possess provincial peculiarities of 
interest, but that these are not by any means identical with the 
features emphasized by Becke and others as characterizing the Pacific 
kindred. 
The factors of magmatic differentiation are then reviewed. The 
aschistic and diaschistic magmas of Brogger and the ‘ dyke rocks’ of 
Rosenbusch are discussed; and it is contended that certain dyke 
rocks of Colorado show a notable exception to the rule postulated by 
Rosenbusch. The conclusion is reached that the sharp distinction 
between the two ‘dyke rock’ groups is a purely arbitrary one, resting 
on an unproved hypothesis. 
A discussion on the classification by eutectics follows, and the 
writings of G. F. Becker and J. H. L. Vogt on this subject are 
criticized. The view that graphic, spherulitic, and felsitic textures 
are characteristically eutectic is considered to be incorrect, and it is 
contended that magmatic classification by eutectics is fundamentally 
weak, because it rests on hypothesis, because it does not apply to all 
rocks, and because it does not allow for the entire magma of most 
rocks. A classification by eutectics may, in the future, be realized ; 
but it seems inevitable that it must be a classification for a special 
interest, not for the general science of petrography. 
The author considers that the distinction between felspathic and 
non-felspathic rocks which has been so prominent in current systems 
is not only unnatural, but is in the highest degree arbitrary. 
The use of texture is then discussed, and it is shown that classification 
by occurrence, as determining texture, or by texture, as expressing 
the broad phases of occurrence, is based on long disproved generalizations 
made from limited observation. The ‘‘ American Quantitative System 
of Classification” is then briefly dealt with, and the following general 
conclusions are formulated :— 
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