INTRODUCTION 



some equally rich ones were formed in similar rock along the 

 lower side of the sills. 



The descriptions of the various mines of the district are 

 intended chiefly to be of aid to those interested in their fur- 

 ther development, but the descriptions also contain details of 

 geology and ore controls that will be of service to those desir- 

 ing a thorough understanding of the deposits. Following each 

 description is a section devoted to a comprehensive analysis 

 of the possibilities for further development. In this section 

 we have considered the known sources of submarginal ore and 

 have also tried to indicate where ore of high grade might per- 

 haps be obtained. As mining in the district was carried on 

 during the last 40 years without adequate exploration and 

 development, the amount of submarginal ore that is readily 

 accessible is small. On the other hand, the geologic structures 

 In some of the mines, when considered along with the factors 

 responsible for the localization of the rich ore bodies, indicate 

 that intelligent and aggressive development, supported by ade- 

 quate funds, can be expected to reveal new ore bodies. Such 

 ore bodies, if as rich as those previously mined, would be 

 minable even during periods when the value of quicksilver 

 is low. 



A history of the New Almaden mine extends through a pe- 

 riod of more than 100 years. The cinnabar was first used by 

 Indians as a pigment. While California was under Mexican 

 rule, the mine was developed according to ancient Spanish 

 mining methods. Later, after the admission of California to 

 the United States, title to the property was obtained by an 

 American mining company through a series of legal battles 

 fought through State and Federal Courts and finally settled 

 by international arbitration. The mining history is exception- 

 ally interesting because it begins with primitive methods and 

 extends through a period when new techniques for mining the 

 ores and new methods of recovering mercury from them were 

 first introduced at New Almaden ; in essence, it provides a his- 

 tory of mining and metallurgy for the mercury mining indus- 

 try of the United States. 



INTRODUCTION 

 SCOPE OF THE REPORT 



The first recognition of quicksilver ore in the United 

 States was made in 1845 in the New Almaden district, 

 and since then the district has yielded nearly 40 per- 

 cent of all the quicksilver produced in this country. 

 Most of its production came from the famous New 

 Almaden mine, which is one of the great quicksilver 

 mines of the world, but the district contains other 

 formerly productive mines, including the Guadalupe, 

 which ranks sixth among the quicksilver mines of 

 California. In spite of its prominence, the district 

 had been little studied until a comprehensive geologic 

 investigation was made during and after World 

 War II by the Geological Survey. The prime pur- 

 pose of this study was to determine whether this 

 district should be regarded as exhausted, or whether 

 it may still contain hitherto unknown ore bodies. 

 Because the New Almaden quicksilver deposits are 

 similar in origin and environment to many others in 

 California, parts of this report may be applied equally 



well to several other quicksilver districts in the State. 

 The mines lie in an area of unusual sedimentary 

 and volcanic rocks making up the Late Jurassic and 

 Cretaceous Franciscan group, which is exposed in, or 

 believed to underlie, at least 30,000 square miles in 

 California. As the parts of the district containing 

 these rocks are of great economic interest, they have 

 been studied more intensively and mapped in greater 

 detail ; other parts containing only younger formations 

 have been mapped and studied less thoroughly. 



LOCATION AND ACCESSIBILITY 



The area mapped for this study as the New Al- 

 maden district includes about 80 square miles in the 

 west-central part of Santa Clara County, Calif., about 

 9 miles south of San Jose and 50 miles southeast of 

 San Francisco. (See fig. 1.) On the General Land 

 Office grid it includes parts of T. 8 and 9 S., R. 1 W., 

 and R. 1 and 2 E., Mount Diablo meridian, and it lies 

 in the northern third of the Los Gatos 15-minute 

 quadrangle. The mines of the district were once served 

 by a branch line of the Southern Pacific railroad, but 

 this line was abandoned and the track removed many 

 years ago; the mines are now reached by good paved 

 and ballasted roads extending from San Jose and Los 

 Gatos. Branch roads lead to many ranches and sum- 

 mer homes remote from the mineralized belt, so that 

 few parts of the district are more than a few miles 

 from a passable road. 



TOPOGRAPHY 



The topography in most of the district reflects the 

 dominant northwesterly trend of the bedrock struc- 

 tures, so that the main ridges and valleys trend north- 

 westward. (See fig. 2 and pi. 1.) In the northern 

 part of the district, however, the bases of the moun- 

 tain ridges have been overlapped by the alluvium fill- 

 ing the southern part of the Santa Clara Valley, which 

 slopes gently northward to San Francisco Bay. The 

 alluvium nearly everywhere separates the northern- 

 most of the three principal ridges in the district from 

 the other two, and alluvial tongues extend up some of 

 the valleys in the next ridge to the south. The other 

 two main ridges are less distinctly separated, because 

 the longitudinal valleys between them lie above the 

 general slope of the Santa Clara Valley and they are 

 therefore sharply incised and devoid of alluvial filling. 



The northernmost ridge the Santa Teresa Hills 

 emerges from the alluvium at an altitude of about 

 200 feet in its western end and attains a maximum 

 height of 1,150 feet at Coyote Peak near the eastern 

 edge of the district. The next ridge to the south, 

 across the valley of Alamitos Creek, is of special in- 



