410 ABSTRACTS 



Nevada City, and Banner Hill; three corresponding maps showing the 

 economic geology, and three others giving structure sections. 



These maps, on a scale of about four inches to the mile, have been 

 prepared to illustrate the detailed structure of the gold-mining regions 

 in the vicinity of Nevada City and Grass Valley. Each of them com- 

 prises an area three miles wide by four miles long, the total area being 

 nearly thirty-six square miles. The Nevada City and Grass Valley areas 

 fall within the boundaries of the Smartsville atlas sheet, while the larger 

 part of the Banner Hill area falls within those of the Colfax atlas sheet. 

 The relief is that common to the middle foothill region of the Sierra 

 Nevada — that is, the surface is a very irregular and undulating plateau 

 deeply trenched by the canyons of the recent river systems. 



Sedimentary rocks, chiefly referred to the Calaveras formation, 

 occupy small, usually long, narrow areas imbedded in the predomi- 

 nating igneous masses. Granodiorite occupies a large part of the 

 Nevada City and Banner Hill districts, while a small massif of the 

 same rock is found in the Grass Valley district. Large areas of dia- 

 base, porphyrite, and brecciated forms of these rocks surround and 

 separate the granodiorite areas. In the southwestern part of the 

 Nevada City district and the northeastern part of the Grass Valley, a 

 large and complicated viassif \s found, consisting in part of diorite, in 

 part of gabbro, pyroxenite, and serpentine. 



The slates of the Calaveras formation are the oldest rocks. Next 

 younger are the diorites, gabbros, and serpentines. Still later are the 

 diabases and porphyrites ; and the intrusion of granodiorite closed the 

 succession of igneous rocks. The bed-rock series is, as usual, in part 

 covered by several hundred feet of Neocene gravels, and rhyolitic and 

 andesitic tuffs, the gently sloping top of the andesitic ridges forming 

 a principal feature of the landscape. 



The Neocene auriferous gravels have been extensively worked in 

 the Nevada City and Banner Hill districts, both by the drifting and 

 the hydraulic processes, and considerable ground still remains which 

 probably can be profitably worked. The gold-quartz veins are numer- 

 ous and belong to several distinct systems. They are found in all of 

 the formations represented on the sheet, and generally cross the con- 

 tacts without change. In the Banner Hill district the veins are narrow 

 but rich, and have a general east-west direction and a northerly or 

 southerly dip. In the Nevada City district the quartz veins have a 

 general north-south direction and an easterly dip of about 45°. Large 



