G. J. Brush on Durangite. 179 
fertile they are soon covered with vegetation which is often bur- 
ied in situ by temporary advances in the glacier. Besides, in- 
sects die on the ice, and falling leaves are blown into its crevasses. 
If the theory of submergence be preferred, then aquatic fossils 
should be found, and one would hardly suppose that any vio- 
lence of the currents would wholly destroy them. If we ad- 
here to the theory of submergence we must of course refer the 
erosion and smoothing of the rocks to the violent action of 
floating ice. Without venturing any dogmatic opinion on 
these difficult points, it seems to me that the phenomena 
favor the following inferences, 
. The western boulder drift is a stratified water deposit. 
2. A breadth of some 200 miles along its southern border 
rests on the Pliocene soil, small patches of which are also found 
a hundred miles north of Chicago. 
3. The drift does not merge directly into the loess but is 
separated from it by a bed of soil. 
- The waters of the drift had at the north a great velocity, 
and a depth of over 800 feet above Lake Michigan, but as it 
passed southward the water lost its swiftness and deposited its 
sediment, 
5. A considerable depth of stiller water remained after the 
cessation of the great current, long enough to deposit the thin 
Stratum of orange loam. : 
. The final retirement of the drift waters from this region 
was rather sudden, the water falling at least 1300 feet without 
leaving any beach lines, 
81 Monroe st., Chicago, June 7, 1869. 
Art, XIX.—Contributions from the Sheffield Laboratory of 
Yale College. No. XX.—On Durangite, a jfiuo-arsenate 
Jrom Durango in Mexico ; by Guonce J. BRusu. 
Tux stream-tin deposit of Durango in Mexico is well known 
among American mineralogists for the remarkable specimens 
Quite recently a bright orange-colored mineral has 
been found at that locality, and I am indebted to Mr. Henry 
G. Hanks, of San Francisco, for the opportunity of investiga~ 
ting it. Mr. Hanks generously placed at my dis three 
Small crystals, with the request that I would examine the min- 
eral and describe the species should it prove to be new. 
The crystals have a rhombic aspect; color, bright orange-red, 
almost exactly the shade of the Uralian chromate of lead ; 
