INTRODUCTION 



9 



their real position to their apparent position in rela- 

 tion to the contours was necessary. . To get the best 

 fit to the topography, and yet retain the proper inter- 

 relationship of the various geologic units, most of these 

 adjustments were made in the field. 



On the larger scale maps of the areas over the New 

 Almaden mine (pi. 3) and the Guadalupe and Sena- 

 tor mines (pi. 14), the geologic contacts are much 

 more accurately placed than on even the more detailed 

 part of the district map. On each of these larger 

 scale maps the contacts were controlled by stadia shots 

 in those parts where new topography was sketched ; 

 elsewhere they were as closely placed as possible by 

 using Brunton compass bearings on known points on 

 the topographic base, and by careful pacing. As the 

 larger scale maps are accurate, no adjustment of con- 

 tacts to obtain agreement with the topography was 

 necessary. 



The geologic maps of the underground workings of 

 the New Almaden mine are based on data from many 

 sources, and, as on the surface maps, the accuracy of 

 the geology shown on them varies from place to place. 

 As might be expected in a mine more than 100 years 

 old and containing more than 30 miles of workings, 

 many parts are inaccessible. When the mine was 

 mapped in 1944-45, practically all the workings be- 

 low the 800 level were flooded, and the most ancient 

 workings, which lie near the surface, were largely 

 caved or filled. However, between 1865 and 1904 the 

 Quicksilver Mining Co. had prepared a very accurate 

 and complete planimetric map of the open workings 

 at a scale of 40 feet to the inch. This map, which was 

 an outstanding example of mine mapping for its pe- 

 riod, is preserved on a large roller in a maphouse on 

 Mine Hill. It has been used as a base for all inacces- 

 sible workings, and data from a few even older maps 

 were incorporated to add workings which apparently 

 were inaccessible by 1865 ; but even so, a few workings 

 that are known to exist have been omitted for lack of 

 any kind of map. The mining after 1904 was rather 

 irregular and consisted mostly of cleaning out and 

 enlarging old stopes; as a result the company map. 

 although very good for the access workings, was not 

 reliable in the stopes. To verify the company's map- 

 ping and obtain control points in the stopes, the Sur- 

 vey field parties ran several miles of closed traverses 

 by means of tape, Gurley compass, and planetable, and 

 also mapped the walls of all accessible workings. 



Altitudes throughout the mine presented greater 

 problems than did the planimetric control because only 

 a few points of known altitudes are indicated on the 

 company maps. Because the shapes of gently inclined 



686-671 i 



geologic contacts, particularly in the stopes, are of 

 great importance in establishing the structural con- 

 trols for the ore bodies, it was necessary to obtain rea- 

 sonably accurate vertical control throughout the mine. 

 For those parts of the mine that were open, levels were 

 run by means of closed foresight and backsight tra- 

 verses, using 6-foot folding wooden carpenter's rules 

 as handy level rods. In the inaccessible areas altitudes 

 were generally known for all levels at the shafts and 

 at important junction points, but elsewhere they were 

 estimated on the basis of the exceptionally steep gra- 

 dient of iy 2 feet per hundred at which levels were 

 run during the period of mining. 



As is discussed in greater detail on pages 110-121, 

 the ore-controlling contacts of the open parts of the 

 New Almaden mine are generally irregular in both 

 strike and dip, and in many places they dip at low 

 angles. To show their true shape in the stopes, they 

 have been contoured at known elevations at regular 

 intervals of from 5 to 10 feet by means of hand level- 

 ing from points of known altitudes. Detailed maps 

 of the New Almaden mine prepared by using these 

 methods are of great value to a few persons, but they 

 were not believed to be of sufficient value to the gen- 

 eral reader to justify publishing them with this report. 

 However, 20 such large maps of the accessible work- 

 ings of the mine at a scale of 40 feet to 1 inch have 

 been placed in open file and can be consulted at the 

 U.S. Geological Survey offices in Menlo Park, Calif., 

 and Washington, D.C., or at the office of the Califor- 

 nia State Division of Mines in the Ferry Building in 

 San Francisco, Calif. The major features of the geol- 

 ogy shown on these maps have been incorporated in 

 the smaller scale composite level maps accompanying 

 this report, and especially suitable parts of them are 

 used herein as text figures. 



The geologic features shown on the maps of the 

 inaccessible workings of the New Almaden mine 

 were compiled from monthly surveyor's records, and 

 although not accurate in detail, they probably serve 

 to give the broad features of the ore control and 

 geology. This could not have been accomplished 

 without the efficient help rendered by Virginia S. 

 Neuschel, of the Survey, who spent 2 months of the 

 summer of 1945 in transcribing the notes from chron- 

 ologic order to an order based on the space relations 

 of the workings and in plotting the data on base maps. 



In the outlying mines, no attempt was made to 

 use the very time-consuming contact-contouring tech- 

 nique, but, instead, the geology was plotted waist high 

 on levels or shown by sections and sketches in ir- 

 regularly floored stopes. 



