8 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



The California State Division of Mines aided in 

 financing a part of the field investigation leading to 

 this report, and the writers are indebted to Dr. Olaf 

 P. Jenkins, who was then Chief of the Division, for 

 his unfailing interest in the geology and economic 

 potentialities of the district. 



Many of the old photographs included with this 

 report were supplied by Mr. Laurence E. Bulmore, of 

 Berkeley, Calif., who has made an extensive collection 

 of photographs and other historical material pertain- 

 ing to the early history of the mine. He also kindly 

 supplied information bearing on the local history dur- 

 ing the period from September 1878 to December 

 1899, when his father, Robert R. Bulmore, was cashier 

 and later general agent for the Quicksilver Mining 

 Co. 



During the field investigations the Survey parties 

 received wholehearted cooperation from the local min- 

 ing companies and mine operators. The owners and 

 operators of the New Almaden and Guadalupe mines 

 made all their records and maps available to the Sur- 

 vey. Mr. C. N. Schuette, manager of the New Al- 

 maden Corp., and Mr. George F. Kirk, operator of 

 the Guadalupe mine, both gave much information 

 about the history of their respective mines and volun- 

 teered all pertinent facts regarding their more recent 

 operations. Mr. H. F. Austin, who operated an in- 

 teresting cinnabar placer deposit in Almaden Canyon, 

 provided information about that deposit. Many other 

 miners and local residents, too numerous to mention 

 individually, helped by their continued interest and 

 assistance to facilitate the investigation. 



MAPPING METHODS 



The methods used in mapping geologic features as 

 exposed on the surface or in a mine depend on a 

 great many conditions, such as amount of exposure, 

 complexity and attitude of structures, character and 

 persistence of rock units; they also depend on the time 

 available and the ultimate objective of the study. For 

 proper evaluation of the resultant maps the critical 

 reader must know something about these factors and 

 about the mapping techniques that were adopted to 

 take advantage of the favorable features and mini- 

 mize those less favorable. Terrains underlain by rocks 

 of the Franciscan group and the closely associated 

 intrusive serpentine bodies present inherent difficulties 

 to the geologist; quicksilver deposits likewise are char- 

 acteristically erratic and irregular, and therefore hard 

 to delinate. The following paragraphs briefly sum- 

 marize the techniques used in the work on the New 

 Almaden district and in the mines. 



Enough time was available for mapping in as much 

 detail as seemed to be justified by the probable useful- 

 ness of the results. Because of the difference in prac- 

 tical application of the various parts of the geologic 

 map of the district (pi. 1), it represents two different 

 methods of mapping. In general, the area containing 

 the mineralized belt, lying to the north of a line be- 

 tween Los Gatos and a point 1 mile west of the south- 

 east corner of the district, was mapped in detail. 

 whereas the rest, except for a few important local 

 areas, was covered only by reconnaissance mapping 

 methods. 



Within the area studied in detail, only the Fran- 

 ciscan rocks and serpentine were believed to be poten- 

 tial ore bearers; all the younger rocks, therefore, wen- 

 examined and mapped less thoroughly. In the Fran- 

 ciscan terrain exposures are poor, being estimated to 

 amount to no more than 0.1 percent of the area, and 

 generally it was not possible to set up a stratigraphic 

 succession or rely on "key beds.'" Every contact was 

 followed as closely as possible, however, by the use of 

 available outcrops and by identifying rock fragments 

 in the residual soil. The detailed part of the map is 

 believed to be in accord with all outcrops and reliable 

 float, although the way in which the outcrops are 

 grouped into rock masses is subject to interpretation. 

 In the area studied by reconnaissance methods no at- 

 tempt was made to follow every contact. In extreme 

 instances, particularly in the south-central part of the 

 district, traverses were so widely spaced as to omit 

 on-the-spot examination of areas more than half a 

 square mile in extent; however, as is indicated by the 

 recorded data on the district map, traverses were gen- 

 erally no more than two thousand feet apart. Conse- 

 quently, some small rock bodies, only a few hundreds 

 of feet or less in area, have doubtless been omitted 

 from the reconnaissance part of the map: but their 

 omission probably does not seriously detract from a 

 reasonable understanding of the regional geology. 



The accuracy of position of a contact as shown on 

 geologic map depends not only on the certainty with 

 which it can be located in the field but also on the 

 precision with which it can be placed on the field map. 

 The topographic base used for the New Almadeii dis- 

 trict mapping was a 2.6 enlargement of the Geological 

 Survey's 15-minute topographic map of the Los Gato- 

 quadrangle, surveyed in l'.M.~> 1(1 and published at a 

 scale of 1:62,500. Aerial photographs were used in 

 the field in conjunction with these enlargements, and 

 much of the geology was first plotted on the photo- 

 graphs, but owing to inaccuracies of the enlarged 

 topographic base, some adjustment of contacts from 



