USE OF SYMBOLS IN EXPRESSING THE QUANTITATIVE 
CLASSIFICATION OF IGNEOUS ROCKS 
WHITMAN CROSS 
In a recent publication F. W. Clarke’ has given, among various 
geochemical data, a new average for the existing chemical analyses 
of igneous rocks of the world, together with other averages for 
continents, countries, or districts. Clarke also gives a table, pre- 
pared by Iddings, showing by names and symbols the quantitative 
classification of magmas corresponding in composition to those 
averages. Speaking of these averages as representing magmas or 
rocks, it is a fact, of no special significance in itself, that many of 
them chance to fall very near boundary lines of several divisions 
of the quantitative system. Consequently the simple names and 
symbols of Clarke’s table fail to show satisfactorily the true syste- 
matic relations of such rocks. These relations may, however, be 
very clearly expressed by means of the elaborated symbols, proposed 
since the publication of Clarke’s paper, by the authors of the 
quantitative system.? The efficacy of these symbols in expressing 
certain relations may be illustrated by showing their application 
to the averages of Clarke. 
As the basis for the calculations to follow and to further dis- 
seminate the interesting averages made by Clarke, they are repeated 
in the accompanying table. Water and several rare or unimportant 
constituents have been omitted by Clarke and the remainders 
calculated to 100 per cent. The capital letters by which Clarke 
designates individual averages in his tables have been retained. 
The normative ratios of the table on p. 760 give the data 
by which rocks of these average compositions may be accurately 
classified in the quantitative system. From these ratios the elabo- 
t “Some Geochemical Statistics,” Proc. Am. Phil. Soc., XLI (1912), 214-34. 
2, W. Cross, J. P. Iddings, L. V. Pirsson, and H. S. Washington, “‘ Modifications 
of the Quantitative System of Classification of Igneous Rocks,” Jour. Geol., XX 
(1912), 550-61. 
758 
