J. D. Dana—Rock notation for Geological Diagrams. 7 
Art. IIL.—On a System of Rock notation for Geological 
diagrams ; by JAMES D. Dana. 
In geological diagrams it is often desirable that the kinds of 
rock should be indicated as well as their position. The follow- 
ing method of accomplishing this purpose is both simple and 
definite, and I think it will be found suited to general adoption. 
I have been led to the subject by the necessity of preparing 
many diagrams to illustrate an account of my recent observa- 
tions on the stratigraphical relations of the Green Mountain 
formations. 
In selecting the symbols, the fact that a sandstone is usually 
represented by a dotted surface suggested for quartz a dot (*); 
and another fact, that a slate or shale is indicated by parallel 
lining, suggested the use not only of such lining for shale or 
slate, but further of a short line like a hyphen (-) for mica; of 
a similar line three times as long (—) for hydrous mica (damour- 
ite, etc.), hydromica or sericite schist being intermediate in 
aspect between a slate and a mica schist. 
To these symbols, I add a cross (+) for orthoclase feldspar ; 
4 cross in oblique position (xX) for a triclinic feldspar or plagio- 
clase ; and the same cross with a line at bottom for a basic feld- 
spar, andesite, labradorite or anorthite, when it is desirable to 
recognize the distinction. 
The other more common of the essential constituents in 
rocks are hornblende, augite, hypersthene, chrysolite or olivine, 
leucite and glass. For hornblende an n is used; for augite, wu; 
for hypersthene when a predominant constituent, y; for olivine, 
v; for leucite, 7; for glass, o. 
To distinguish thin schistose structure ,or slaty bedding, a 
rule is inserted between the lines of symbols; for thick schistose, 
gneiss-like, a rule is inserted at intervals o to 4 lines of — 
symbols ; and for massive structure where bedding is recog- 
nizable, a rule at intervals of 6 or more lines of symbols or 
without rules. 
0 attempt is made to distinguish varieties of the different 
rocks dependent on accessory ingredients, as this would make 
the system cumbrous and impracticable. The following figures 
illustrate the method: 
