352 Association of American G'eologists and Naturalists. 
Pink (lake) was proposed for granitic rocks—greens for serpentine, 
greenstone and trap; the first being the light green, the last the dark- 
est; or if particular greens for each be thought preferable, terre verte 
may represent serpentine; sap green, greenstone; Prussian green, 
trap ; reddish grey, for trachyte; light grey, for modern lavas; reds 
with yellow spots, for porphyry; grey with white spots, amygdaloid. 
A purple color composed of the lake of the granite and the indigo of 
the lowest fossiliferous, was thought to be the most appropriate color for 
the metamorphic rocks. If it should be thought desirable to distinguish 
the different members of the metamorphic group, neutral tint might re- 
present gneiss; native ochrous purple, mica slate; archal, hornblende 
schist ; and the mixture of indigo and lake of the period clay slate and 
killas. Ultramarine, or ultramarine and lake, might represent metamor- 
phic limestone. For the carboniferous rocks, considered by some an 
independent formation, burnt sienna was recommended, which having 
a decidedly red hue, would give to this formation the tint of the period 
to which it belongs. 
In detailed sections and charts the same general system should be 
adhered to. If a variation of the tint should not be found sufficiently 
distinct in such cases, then sandstones and siliceous deposits of the first 
period may uniformly be indicated by chrome yellow ; those of the sec- 
ond period, by yellow ochre or the red color of the period; those of the 
third period, by gamboge yellow ; limestones of the first period may be 
represented by indigo; those of the second period, by Prussian blue ; 
those of the third period, by cobalt blue. If possible, the demi-tints 
should not predominate, but rather the primitive color of the period. 
All the primitive tints being transparent colors, they can be more ea- 
sily laid on in a neat, delicate and uniform manner, than the opaque 
mineral pigments; and, if desirable, they may be glazed on the top of 
the lithological tints, and the general hue of the period thus imparted. 
Thus, according to the plan proposed for detailed geological sections, 
blue will always indicate limestones or calcareous deposits ; greys, ar- 
gillaceous strata; greens, argillo-calcareous ecpaelss dark grey or 
black, carbonaceous beds. 
In addition to this system of coloring, still further to facilitate the re- 
cognition of formations, a system of symbols was also proposed, on a 
similar plan to chemical symbols, taking the first letter of the name of 
a formation to represent it, and, when that was appropriated, the first 
and second, or when two or three words are employed, the first letter 
of each word, thus: 
Granite, G. Gneiss, Gn. 
Metamorphic rocks, M. Mica slate, M. 8. 
