HOUTHORS” ABSTRACTS 251 
amphibolites. Small masses of serpentine are sometimes found in the 
amphibolite; others appear intimately connected with gabbrodiorite. 
Superficial flows of andesitic tuffs and breccias cover the older rocks. 
The larger part of these flows has been eroded. The remaining 
masses form sloping tables in the lower foothill region. Auriferous 
gravel channels are found in places below these volcanic rocks. At 
an elevation of 300 feet the andesite is underlain by clays and sands 
of the lone formation, deposited in the gulf which in Neocene times 
skirted the foothills of the Sierra Nevada. The western part of the 
tract is largely covered by early Pleistocene deposits of gravel, sand, 
and hardpan. 
Economic geology. Neocene auriferous gravels have been worked 
to some extent east of Rocklin and south of Auburn. The Pleistocene 
gravels in the foothills have been very rich in gold, but are now mostly 
exhausted. . At Folsom large masses of Pleistocene gravels are still 
worked. Auriferous quartz veins have been extensively worked 
between Ophir and Auburn. Small veins are occasionally worked 
near Clarksville and in the vicinity of Pilot Hill. 
The central mass of granodiorite affords excellent building stone. 
Limestones occur chiefly as lenses in amphibolite at many places along 
the eastern border. The soils of the foothill region are residuary in 
character, while the western part of the tract is occupied by deep 
alluvial and sedimentary soils. 
U.S. Geologic Atlas. Folio 7, Pikes Peak, Montana, 18094. 
Tuis folio consists of four and-a-half pages of text, signed by Whit- 
man Cross, geologist, a topographical sheet (scale 1: 125,000), a sheet 
of areal geology, one of economic geology, and one of structure sec- 
tions ; followed by a special description of the Cripple Creek mining 
district, consisting of one page of text on the mining geology, by R. 
A. F. Penrose, Jr., and a map (scale 1: 25,000) showing the economic 
geology of the district. 
Geography.—The district embraces an area of 931.5 square miles, 
between meridians 105° and 105° 30’ and parallels 38° 30’ and 39°. 
In its eastern half lies the crest of the granitic Colorado Range, which 
extends from Manitou Park though Pikes Peak to the southern end of 
the range, where it sinks to the level of the plains. The western por- 
tion of the area is a plateau of granite and volcanic rocks, lying between 
8000 and 10,000 feet in elevation, penetrated on the south by deep 
