312 
lower parts of these lower deposits strike into the 
Arapahoe and Denver formations of the Denver 
Basin, and it appears that those formations are 
equivalent to part of the lower division of the 
“¢Monument Creek.’’ 
Dark Scale of Hardness: ALFRED C. LANE. 
The hardness of a mineral is its resistance to 
shearing stress. Like other properties of minerals, 
it may differ in different directions. When two 
similar surfaces are rubbed together, the softer 
mineral leaves a powder (streak) on the other. 
In order to be sure which mineral gives the streak, 
it is at times convenient to have besides the com- 
mon Mohs scale of hardness, composed of light 
minerals, a ‘‘dark scale of hardness’’ of minerals 
whose color and streak is dark, especially in teach- 
ing. For such minerals, the following properties 
are desirable: quickly recognizable, easily obtain- 
able, hardness uniform. The following minerals 
have been used by the writer: (1) graphite, with 
1 good cleavage, at one extreme in the white scale, 
at the other extreme in the black; (2) stibnite 
(Sb.8,), with 2 good cleavages, bladed; (3) galen- 
ite (PbS), with 3 good cleavages; (4) iron (use 
soft wire nail), magnetic, ductile; (5) niccolite 
(NiAs), characteristic color, no cleavage; (6) 
magnetite (Fe,O,), magnetic, brittle. The min- 
erals mentioned seem fairly satisfactory. Above 
six, the author does not know whether spinel or 
some other mineral would be more desirable, but 
as there are comparatively few minerals con- 
cerned, it is not important. 
Demonstration of Relative Refraction: ALFRED C. 
LANE. 
The method of determining relative index of 
refraction developed by Exner, Becker and 
Schroeder van der Kolk? may be demonstrated to 
a class as follows: A large beaker of water placed 
just a little to one side of a window or other 
source of light will show a bright streak on the 
farther side, having a higher index than the sur- 
rounding medium. -A test tube full of air placed 
in the beaker will show a bright streak of total 
reflection on the nearer side, the air within having 
a smaller index than the surrounding medium. 
A Stratigraphic Study of the Appalachian and 
Central States with Reference to the Occurrence 
of Oil and Gas: Gro. H. ASHLEY. 
It has always been supposed by the oil and gas 
men that the Appalachian region, extending from 
Pennsylvania to Alabama, was a_ stratigraphic 
1Report Michigan Geological Survey, Vol. VLI., 
p. 154. 
SCIENCE 
[N.S. Von. XXXV. No. 895 
unit, and it has been a mystery that eastern Ken- 
tucky, Tennessee and Alabama should not yield as 
much oil and gas as Pennsylvania or West Vir- 
ginia. The mystery largely disappears when a 
comparative study is made of the stratigraphy of 
the Appalachians as a whole. The paper compares 
graphically the stratigraphy of the southern and 
northern Appalachians and the central states with 
reference to the occurrence of oil and gas. 
The paper was discussed by H. B. Kiimmel. 
Granularity Limits in Petrographic-microscopic 
Work: FRED E. WRIGHT. 
In this paper the petrographic microscope is 
treated as a measuring device for the exact deter- 
mination of the optical properties of crystal plates, 
especially of minute crystal fragments and of 
erystallites. The methods now available for the 
purpose are considered briefly with special refer- 
ence to their accuracy and applicability to the in- 
vestigation of fine-grained silicate preparations. 
Attention is directed in particular to the lower 
granularity limits at which satisfactory measure- 
ments of the different optical properties of a 
mineral grain can still be made. 
The Arkansas Diamond-bearing Peridotite Area: 
L. C. GLENN. 
Evidence will be offered of the circulation of 
presumably thermal waters about the margin of 
the original pipe described by Branner. A sup- 
posed extension of the peridotite area proves to 
consist of disintegrated peridotite mixed inti- 
mately with well-rounded quartz sand and occa- 
sional water-worn chert pebbles and was evidently 
water-laid. Indications of the age of this material 
will be given, narrowing down the period within 
which the extrusion of the peridotite must have 
occurred. 
The paper was discussed by A. H. Purdue. 
Resins in Paleozoic Coals: Davin WHITE. 
Resins are present in most coals, except possibly 
those of the highest grades, the amount depending 
in general on the degree of concentration (resid- 
ual) resulting from the decay and reduction of the 
attending plant structures. Interesting examples 
of megascopic resins in coals from the Carbon- 
iferous of the Mississippi Valley and Montana 
indicate the presence of resin in the Paleozoic 
coals in proportions probably as large as in the 
coals of the Mesozoic and Tertiary. 
Onyx Deposits in East Tennessee: C. H. Gorpon. 
The existence of onyx deposits in east Tennessee 
has been known for a long time, and attempts have 
been made at recurrent intervals to utilize the 
