REVIEirS AXn ABSTRACTS 879 



meridians 120"' and 121 30' and the parallels 39° and 39° 30', 

 and embraces about 925 square miles, comprising a part of the foothill 

 region of the Sierra Nevada. The elevation ranges from 50 feet above 

 sea level in the northwestern corner to over 4000 feet in the north- 

 eastern corner. The topography is characterized by a number of par- 

 allel ridges, running in a north-northwest direction. The northeastern 

 part has more the character of an irregular and undulating table-land. 

 Through the ridges and the plateaus the watercourses have cut deep 

 and narrow canyons. The Yuba River, with its branches, drains the 

 larger part of the district. Honcut Creek on the north and Bear 

 River on the south, are the only other streams of importance. 



Geology. — Sedimentary formations occupy comparatively few areas 

 in the district, all of which has been tentatively referred to the Cala- 

 veras formation, no fossils having been found in them. They consist 

 of slates and quartzitic sandstones, usually with northerly strike and 

 steep easterly dip. Diabase and porphyrite occupy large areas in the 

 central and southern parts, as well as intrusive masses of granodiorite 

 and gabbrodiorite. Amphibolites, resulting from the dynamo-meta- 

 morphism of diabase, gabbro, and diorite, also occur in several places. 

 The rocks of the district are principally massive, in contrast to those 

 of the districts adjoining on the south and east. However, two lines 

 traverse it along which extensive metamorphism has taken place and 

 schistose rocks have been developed. The superjacent rocks, resting 

 unconformably on the older series, consist of Neocene river gravels 

 together with beds of andesitic and rhyolitic tuffs. Comparatively 

 small areas of these remain, the larger part having been carried away 

 by erosion. Pleistocene shore gravels and alluvium occupy the south- 

 western corner. The lone formation is not well exposed in this dis- 

 trict, being in part covered by Pleistocene deposits, in part removed 

 by erosion. 



Economic Geology. — Important and rich Neocene gravel deposits in 

 this district have been worked at Camptonville, Nevada City, North San 

 Juan, Badger Hill, French Corral, and Smartsville. Gold-quartz veins 

 occur scattered throughout the area, but by far the most of them are 

 found in the immediate vicinity of Nevada City and Grass Valley. These 

 districts are among the most important of the gold-mining regions in 

 California. Many of the rocks of the district are adapted for building 

 purposes. The only one in extensive use is the granodiorite near 

 Nevada City. The often deep red soils in the foothill region are of 



