JOT MOIS UBS IORACTES, 977 
General geology.—The alluvial lands consist of sands, clays, and 
gravels, deposited by the shifting currents of the streams. ‘The foot- 
hill region of the northeastern corner is principally occupied by the 
gravels of Pleistocene and Neocene age. The area composed of the 
bedrock series of the Sierra Nevada is small and consists of diabase 
and porphyrite. The mountain group of the Marysville Buttes is an 
extinct volcano of probably late Neocene age, the internal structure of 
which is to a certain extent laid bare by erosion. The eruptive rocks 
of the buttes are andesites and rhyolites. In describing the structure 
of the group three parts may be distinguished: First, the central core 
of massive andesite and rhyolite; second, the upturned sedimentary 
rocks surrounding the massive core, evidently brought into their 
present position by the force of the ascending lavas; the sediments 
are of Eocene and Neocene age; third, the external ring of tuffs and 
breccias. The feature of greatest interest in connection with the 
Marysville Buttes is doubtless the presence of upturned sediments 
around the central core. 
Economic geology.—The shore gravels in the northeastern corner 
contain some gold and have been washed superficially. Somewhat 
auriferous gravels are also found in the upturned sediments of the 
Marysville Buttes. Coal and natural gas have been found in small 
amounts in the Marysville Buttes. 
Lowa Geological Survey. Vol. 1V., Third Annual Report, 1894, with 
accompanying papers. SAMUEL CaLviN, State Geologist, Des 
Moines, 1895. 
The fourth volume of the reports of the Iowa Geological Survey has 
been issued and distributed. So far as size and general make-up are 
concerned this volume is uniform with those previously issued. About 
fifteen pages are devoted to Administrative Reports and the remainder of 
the volume, which contains 467 pages altogether, is devoted to county 
geology. The geologists of lowa have adopted the county as the areal 
unit for final geologic mapping and report. ‘The boundaries of 
counties are definitely located and generally known. The county is one 
of the organic units of civil government. Its inhabitants are bound 
together by common purposes, and have a common pride in its 
resources and in whatever promotes its welfare. Definite information 
