ROCK CLASSIFICATION ON THREE CO-ORDINATES" 
ALEXANDER N. WINCHELL 
There are many published classifications of igneous rocks, and 
their very number and variety are an indication of the difficulties 
inherent in the problem of constructing a satisfactory schematic 
arrangement—difficulties which are due largely to the fact that 
rocks are not chemical units, but mechanical mixtures, whose pro- 
portions may vary almost indefinitely in a great many ways. 
It is generally admitted that at present the most satisfactory 
classifications are based upon, first, mineral (or chemical) compo- 
sition, and, second, texture or geologic mode of occurrence. It is 
to be hoped that with advancing knowledge classifications may be 
based largely upon the principles of eutectics and the method of 
genesis of igneous types; at present the use of such conceptions as 
bases of classification may be recognized as desirable, but must be 
held in abeyance as impossible without more information. 
A classification of igneous rocks based almost wholly upon 
chemical composition has been worked out in detail by Cross, 
Iddings, Pirsson, and Washington.? It has come into use gradually 
by an increasing number of petrologists, but its service to science 
is restricted by two facts: first, it commonly ignores the actual 
mineral composition in favor of an imaginary mineral composition 
known as the ‘“‘norm,’’ and, second, it cannot be used until the 
chemical composition’ of the rock is known. Its great advantage 
lies in the fact that it reveals chemical characteristics and relation- 
ships with fidelity and clearness. 
The classification of Rosenbusch,* which has been developed 
' Published with the permission of the Director of the U.S. Geological Survey. 
2A Quantitative Classification of Igneous Rocks. 1903. 
3 A microscopic determination of the quantitative mineral composition may be 
used as a substitute for a chemical analysis only when the norm and the mode are 
the same. 
4 Elemente der Gesteinslehre. 1910. 
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