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GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



of serpentine appear along the contact, and the crumpled volcanics 

 include fragments of chert and limestone. East of the bridge a few hun- 

 dred yards, the massive greenstones contain amygdaloidal horizons and 

 represent quiet lava flows rather than the pyroclastic beds seen to the 

 west. 



Two miles north of Bear River is Higgins Corner. A road connecting 

 with the hamlet of Wolf leads off to the west and another to the east leads 

 to the Combie Dam and Reservoir on Bear River. A prominent outcrop 

 of Calaveras chert forms a reef just west of the highway 2.3 miles north 

 of the south fork of Wolf Creek. This reef is almost vertical in attitude 

 and is associated with greenstones of uncertain age. This is one of the few 

 exposures of chert to be seen close to Highway 49 south of the Yuba River. 



The Lime Kiln or Jones ranch was the site of a series of limestone 

 quarries and kilns where lime for mortar was prepared at a very early 

 date. Traces of the old workings have been almost obliterated, but par- 

 tially burned limestone marks the old kiln sites. The limestone bodies are 

 small and are now largely masked by the soil mantle. Chemical analysis 

 of the limestone shows it to be of excellent grade. The Lime Kiln ranch 

 is located three miles west of Highway 49 via dirt road. The turnoff is 

 5.3 miles north of Higgins Corners and is marked by a white sign. 



Three miles beyond the chert reef the greenstone bedrock is invaded 

 by granodiorite. The contact is plainly visible in a roadcut on the west 

 side of the highway. The dark minerals in the granodiorite near the con- 

 tact have been altered to chlorite and green amphiboles, and the grano- 

 diorite is almost as green as the meta-andesite series which it invades. 

 A feldspar porphyry is present close to the contact which does not appear 

 to be related to either the meta-volcanic series or the granodiorite, and 

 may have been brought up from below by the intrusion. The porphyry is 

 composed of abundant large plagioclase phenocrysts averaging 3 mm. in 

 diameter set in a fine-grained black groundmass. 



The change in bedrock from meta-volcanics to granodiorite is almost 

 immediately reflected by the vegetation. Heavy growths of manzanita 

 cover most of the granodiorite bedrock with a dense gray-green mantle. 

 The manzanita thickets are most conspicuous in the vicinity of La Ban- 

 meadows. 



Grass Valley is probably the most beautiful active mining camp in 

 California. Mining camps the world over are notoriously ugly and uncom- 

 fortable places to live, but Grass Valley is set in a well watered coniferous 

 forest of great beauty. Many stands of large trees have been spared the 

 loggers axe and tower in dark green borders about broad meadow-lands. 

 The entire aspect of suburban Grass Valley is parklike. Even the mine 

 dumps and buildings are more or less masked by trees so that the scenery 

 suffers little by their presence. The business district of town is much like 

 that of other Sierran towns. 



Grass Valley is connected to the overland route through Reno via 

 Highway 20 which joins Highway 40. A railroad once connected Nevada 

 City and Grass Valley to the transcontinental route of the Southern 

 Pacific but the tracks have been removed. Aside from explorations of 

 Spanish Americans of which there is no record, Grass Valley was first 

 visited by French emigrants in 1846. Gold miners from Oregon spent con- 

 siderable time there in 1848 but the first permanent settlers who were 

 emigrants from the east arrived in 1849. Historical spots in Grass Valley 

 are numerous. The careers of such famous names as Lola Montez and 

 Lotta Crabtree are closely associated with the history of the town. Its 

 lode mines constitute the most productive group of gold properties in 

 California and rank among the richest in the nation. 



The geology of the Grass Valley mines differs greatly from that of 

 the Mother Lode mines. Very little large scale faulting is in evidence in 

 the Grass Valley district. The Mariposa slate is largely absent and even 

 Calaveras rocks usually do not contain ore. The main veins dip on an 

 average of 35 whereas most Mother Lode veins are steeply dipping. The 

 minor cross veins are usually not mineralized except at their intersections 

 with main veins. The wall rocks of the Idaho-Maryland and Spring Hill 

 mines are principally gabbro and serpentine ; those of the Empire Star 

 and Golden Center are granodiorite, meta-andesite, diabase, and Cala- 

 veras schist. Vein forming minerals from the Grass Valley district include 

 ankerite, native arsenic, arsenopyrite, chalcopyrite, chromite, epidote, 

 galena, gold, magnetite, mariposite, pyrite, pyrrhotite, sphalerite, and 

 rarely molybdenite, scheelite, hessite, and altaite. The latter two are tellu- 

 rides of gold and lead respectively. Wall rocks in the district are heavily 

 watered above the 1500 foot level and water flows into the lowermost 

 workings necessitating use of elaborate pumping systems. Pumps which 

 handle 3000 gallons per minute are used in the wettest spots. Although 

 exceedingly humid, Grass Valley mines are among the coolest in the 

 world. The temperature increase or geothermal gradient below ground is 

 less than 1 F. per 100 feet of depth. 



The largest mining operation in Grass Valley is that of the Empire- 

 Star Mines Company, Ltd. The Empire-Star is a consolidation of the 

 major North Star, Pennsylvania, and Empire mines and a host of lesser 

 workings. The Empire has been operated continuously since 1851. Some 

 of the other shafts have been shut down for short periods at various times 

 since discovery. The workings of the Empire-Star total more than 200 

 miles in length making it one of the most widespread mines in existence. 

 It has been mined to an inclined depth of more than 11,000 feet or a 

 vertical depth of over a mile. The total production of the Empire-Star 

 group has been in excess of $120,000,000. For the locations of the various 

 shafts of the Empire-Star and other Grass Valley mines see the accom- 

 panying map. 



