ISTOVEMBEE 29, 1895.] 



SCIENCE. 



Ill 



Economic Geology. — ^The JSTeocene River 

 channels, with very highly auriferous 

 gravel, are exposed and mined at several 

 places in the area, for instance, at Todd's 

 Valley, near Georgetown, and in the vicinity 

 of Placer ville. Many and important auri- 

 ferous quartz veins are found in the area. 

 The principal ones occur along the belt of 

 Mariposa slates previously mentioned, and 

 form the northern end of what is usually 

 referred to as the Mother Lode of Califor- 

 nia. Passing by ISTash ville and Placcrville, 

 the vein is almost continuous up to the 

 northern part of the area, where it splits up 

 into several branches, which die out before 

 reaching the northern border. Important 

 veins are, however, also found both to the 

 east and west of this belt. ISTear the east- 

 ern line lies the important mining district 

 of Grizzly Flat. 



There are practically no alluvial soils in 

 the area. The deep soil on the summit of 

 the ridges is always a residual soil, formed 

 by the decomposition of the rocks in place. 



FOLIO 5, SACRAMENTO, CALIFOENIA, 1891. 



This folio consists of Ih pages of text de- 

 scriptive of the Gold Belt and 1\ pages de- 

 scriptive of the Sacramento tract, signed 

 by Waldemar Lindgren, geologist, and G. F. 

 Becker, geologist in charge; a topographic 

 map (scale 1:125,000) of the tract, a sheet 

 showing the areal geology, another showing 

 the economic geology, and a third exhibit- 

 ing structure sections. 



Topography. — The Sacramento tract in- 

 cludes the territory between the meridians 

 121° and 121°30' and the parallels 38°30' 

 and 39°, and contains 925 square miles. 

 The western half of the tract embraces a 

 part of the Sacramento Valley, while the 

 eastern half contains the first foothills of 

 the Sierra Nevada. The elevation ranges 

 from 30 feet above sea level at Sacramento 

 to 2,100 feet in the northeastern corner of 

 the tract. The foothill region forms a slop- 



ing and undulating table land, through 

 which the American River has cut a deep 

 and narrow canyon. 



Geology. — A small area of sedimentary 

 slates of the Calaveras formation (Carbon- 

 iferous) occurs in the northeastern corner, 

 and a belt of black clay slates belonging to 

 the Mariposa formation (late Jurassic) is 

 contained in the igneous rocks of the south- 

 eastern part. At Folsom the Mariposa 

 slates are cut off and contact metamorphosed 

 by the granitic rocks of the Rocklin massif. 

 The larger part of the older rocks of this 

 tract is of igneous origin. A large area of 

 diabase and porphyrite is found along the 

 eastern margin. Wide belts of these rocks 

 have been rendered schistose and changed 

 to amphibolites bj^ dynamo-matamorphic 

 processes. Several masses of granodiorite 

 and gabbrodiorite have been intruded into 

 the diabases, porphj'rites and amphibolites. 

 Small masses of serpentine are sometimes 

 found in the amphibolite; others appear in- 

 timately 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 west- 

 ern part of the tract is largely covered by 

 early Pleistocene deposits of gravel, sand 

 and hardpan. 



Economic Geology. — Neocene auriferous 

 gravels have been woi-ked 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 . Aurif- 

 erous quartz veins have been extensively 



