A FORM OF MULTIPLE ROCK DIAGRAMS 
FRANK F. GROUT 
University of Minnesota 
A number of suggestions have been made for rock diagrams, 
designed to show the variation in several constituents through a 
series of rocks, and a modification is here offered of a method 
proposed by Adams.' He used the chemical analysis directly. 
This gave a conspicuous line for silica in nearly every analysis. 
It occurred to the writer that the relative proportions of constitu- 
ents could be seen more clearly if the analyses were recalculated to 
the norms,? in which a larger number of constituents are usually 
present in notable amounts. Since the mode sometimes differs 
from the norm, this method is of course subject to any criticism of 
the norm as a method of stating rock composition; but it has some 
advantage over the simple plotting of chemical constituents. 
Furthermore, the method applies to the mode almost as well as to 
the norm. Those who do not like the norm can measure or calcu- 
late the mode, but in this case relatively few constituents ordinarily 
attain prominence. It will be recalled that a single feldspar in the 
mode may be three in the norm, and that certain ferromagnesian 
minerals in the mode may be divided in the norm. 
As a further modification of Adams’ method the writer finds it 
desirable not to plaster the individual rock diagrams together, 
but to clamp them into position leaving them free for rearrange- 
ment, as they are studied from various points of view. ‘Thus in a 
gabbro (see ‘‘A Type of Igneous Differentiation,” p. 627) it was of 
interest to arrange the rocks in what might be called stratigraphic 
position to see if the magnetite showed a tendency to concentrate 
at any special horizon; while in the study of differentiation it was 
*F, D. Adams, “‘A Graphical Method of Representing the Chemical Relations 
of a Petrographic Province,” Jour. Geol., XXII, 689. 
Whitman Cross, J. P. Iddings, L. V. Pirsson, and H. S. Washington, Quantita- 
tive Classification of Igneous Rocks. University of Chicago Press, 1903. 
622 
