AUTHORS’ ABSTRACTS 253 
Eocene—The small lake deposit about Florissant is noted the world 
over for its fossil insects, while fishes, birds, and many plants are also 
found in these thin beds, which are chiefly made up of volcanic ashes. 
The voleanic rocks of the district are numerous and interesting. 
Those of the western portion belong to a great volcanic center south 
of South Park. At Cripple Creek is a local volcanic vent the peculiar 
product of which is the rare rock phonolite. 
- Many points in the geological history of the Colorado Range have 
been brought out by the recent survey, such as the evidence of varying 
relations between land and sea at different periods, shown by uncon- 
formities and by remnants of strata on the granite plateau. The shear 
zones shown by the gneisses, and the observed folds and faults of the 
foothills, bear directly upon the structural history of this portion of 
the Rocky Mountains. 
Economic geology.—The gold-bearing district of Cripple Creek is 
directly connected with the volcanic center. The gold ores are free 
milling near the surface, but pass into telluride smelting ores in depth. 
They occur in veins, chiefly in the volcanic rocks, but occasionally in 
the granite near them. The extreme alteration of the rocks of the 
eruptive center, and the unusual character of the gold veins, have made 
a detailed study of the mining district necessary. A special topogra- 
phic and geologic map on the scale 55455, or nearly 2% inches to the 
mile, has been made, and the ore deposits have been thoroughly exam- 
ined by Professor R. A. F. Penrose, Jr. 
U.S. Geologic Atlas. Folio 9, Anthracite—Crested Butte, Colorado, 1804. 
This double folio consists of three pages of text descriptive of the 
Elk Mountains, by S. F. Emmons; two pages descriptive of the igneous 
formations of the two districts, by Whitman Cross ; four pages descrip- 
tive of the sedimentary formations, by G. H. Eldridge; of each of the 
two districts a topographic map (scale 1: 62500), a map of areal geology, 
another of economic geology, and a third of structure sections ; and 
finally, a sheet showing a generalized columnar section of the two 
districts. 
Geography.—The combined area represented on the two sheets 
covers one-eighth of a degree, lying between the parallels 38° 45’ and 
39’ and the meridians 106° 45’ and 107° 15’, and is about 27% miles 
long from east to west and 17% from north to south. It includes the 
southern third of the Elk Mountain group, which lies between the 
