6 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



terest because it contains all the highly productive 

 quicksilver mines. It is known, in its central part at 

 least, as Los Capitancillos Ridge. At its northwest 

 end it rises abruptly from the Santa Clara Valley to 

 an altitude of about 800 feet, and to the southeast it 

 rises gradually to 1,750 feet on Mine Hill. Farther to 

 the southeast it is more dissected, but, nonetheless, its 

 higher peaks reach approximately the same altitude. 

 This ridge is sharply cut in three places by the trans- 

 verse Guadalupe, Alamitos, and Llagas Creeks, which 

 flow into Santa Clara Valley, and longitudinal tribu- 

 taries of these creeks separate it on the south from the 

 third parallel ridge. 



This third ridge, the Sierra Azul, is a part of the 

 backbone of the California Coast Ranges and is con- 

 siderably higher than either of the others. It extends 

 for several miles with altitudes only a few hundred 

 feet above or below 3,400 feet, but near the western 

 boundary of the district it also is breached by Los 

 Gatos Creek, which flows at grade with the Santa 

 Clara Valley. 



The slopes of the hills vary considerably in steep- 

 ness. In general, the Santa Teresa Hills are fairly 

 subdued, the Los Capitancillos Ridge moderately rug- 

 ged, and the Sierra Azul decidedly rugged. In spite 

 of the general ruggedness of the area, the crests of all 

 the main ridges are characterized by general slopes 

 and local flats. Landslides, ranging in length from a 

 few tens of feet to a mile, are common topographic 

 features on the Los Capitancillos Ridge and the lower 

 slopes of the Sierra Azul. On these same ridges in 

 areas where no distinct slides can be recognized, 

 there are extensive slopes of excessively rocky soil 

 which has moved downslope by creep for long dis- 

 tances. The canyons in these areas are V-shaped, but 

 their troughs are so charged with loose rock that they 

 offer very limited exposures of bedrock. 



CLIMATE AND VEGETATION 



The climate of the district is generally mild but 

 varies somewhat with the altitude. In the Santa Clara 

 Valley the temperature drops a little below freezing a 

 few times each winter, and summer temperatures 

 rarely exceed 100F; the usual daily variation in tem- 

 perature, however, is rather great. Precipitation gen- 

 erally occurs only during the winter and spring, and 

 the wet and dry seasons are reported (Clark, W. ()., 

 1924, p. 40-42, 49) to be more sharply contrasted in 

 the Santa Clara Valley than in any other part of the 

 United States. The precipitation in the valley, which 

 averages 20 inches per annum (Grunsky, 1008, p. 496- 

 543), falls amost entirely as rain; snowfall is so rare 

 that whenever it comes there is a virtual holiday in 



San Jose. In the higher parts of the district the tem- 

 perature range is somewhat greater, owing largely to 

 colder winter nights ; and the average rainfall is about 

 40 inches per year. Some snow falls in the mountains 

 each winter, but generally it melts quickly. 



The vegetation reflects the climatic differences due 

 to altitude, although it is also influenced in a smaller 

 degree by other features, such as northerly or south- 

 erly exposure, kind of soil, and drainage. The broad 

 valleys, which apparently were once carpeted with 

 grass or wild oats and studded with oaks are now 

 largely covered with prune and apricot orchards. 

 Some parts of the lower hills still retain the wild oats 

 and oak trees, but other parts are planted with vine- 

 yards. Higher ground supports a thicker growth of 

 trees with an undergrowth of poison oak in many 

 places. Everywhere, however, there are scattered 

 patches of grassland, and extensive areas, particularly 

 at altitudes above 1,700 feet, are blanketed with 

 "chaparral," a dense head-high growth of shrubs inter- 

 mixed with small trees; the more abundant species 

 are 



Eastwood manzanita ( .Irrhlostaphylo* glandulost) 

 California scrub oak (Quercus dumosa) 

 Wartleaf eeanothus (Ceanothus papUlosus) 

 Chaparral broom (Baccharis consanqvi) 

 Chamise greasewood (Adenostoma fasciculatum) 



The wetter parts of the stream valleys, at altitudes 

 above 2,000 feet, support scattered growths of various 

 conifers. 



The vegetation in some areas is so closely controlled 

 by the underlying rock that the distribution of t la- 

 rocks may be roughly traced by the character of the 

 vegetation. Such special lithologic control of the 

 vegetation is discussed with the appropriate rock de- 

 scription. 



PREVIOUS WORK 



Despite the prominence of the New Almaden dis 

 trict as the foremost quicksilver producer in the 

 United States for more than a century, very little 

 has been published about the geology of either t la- 

 mine or the district. The only lengthy discussion of 

 the geology is the one by G. F. Becker (1888, p. :n<> 

 331, 467-468) in his monograph dealing with most of 

 the domestic quicksilver deposits known in 1888. How- 

 ever, his broad statements concerning the geology and 

 ores and his generalized surface map were based on 

 fieldwork of rather brief duration. The most valuable 

 contribution in Becker's report so far as this district 

 is concerned consists of the excellent planimetric maps 

 of the New Almaden mine workings. Forstner i I'.io. 1 ',. 

 p. 168-187) added a few geologic observations in l'.i:;. 

 but he was much handicapped by the inaccesibility of 



