CONTENTS V 



Page 



FIGURE 27. Photomicrograph of section of altered tachylitic tuff of the Franciscan group. . . 37 



28. Map showing distribution of metamorphic rocks of the Franciscan group and serpentine 38 



29. Photomicrograph of hornblende-albite gneiss 39 



30. Photomicrograph of garnet-hornblende gneiss 40 



3 1 . Polished surface on glaucophane-quartz schist 41 



32. Polished surface on chlorite-lawsonite rock 42 



33. Polished surface on silicified tuffaceous greenstone showing typical "orbicular jasper" structure 43 



34. Photomicrograph of metachert containing needles of crocidolite 43 



35. Blocky serpentine exposed in fresh roadcut 48 



36. Typical boulder strewn surface developed on areas underlain by blocky serpentine 49 



37. View of margin of serpentine mass in the low foothills of the New Almaden district 49 



38. Polished surface on fresh serpentine derived from dunite 50 



39. Photomicrographs of dunite partly replaced by serpentine minerals 51 



40. Photomicrographs of serpentine derived from peridotite 53 



41. Silica-carbonate rock derived from sheared serpentine 59 



42. Polished surface on silica-carbonate rock derived from slightly sheared serpentine 60 



43. Photomicrograph of altered serpentine showing early stage in development of silica-carbonate rock 61 



44. Photomicrograph of magnesite replacement of thick veins of chrysotile 61 



45. Photomicrograph of silica-carbonate rock derived from serpentine that was derived from dunite 61 



46. Photomicrographs of silica-carbonate rock derived from sheared serpentine 62 



47. Diagram showing gains and losses in alteration of serpentine to silica-carbonate rock 63 



48. Interbedded graywacke and shale of Late Cretaceous age in the Sierra Azul 66 



49. Casts of Inoceramus sp 66 



50. One of the smaller quarries in Upper Cretaceous sandstone in the Santa Teresa Hills 67 



51. Dome formed by spheroidal weathering 67 



52. Igloolike rock resulting from spheroidal and cavernous weathering 68 



53. Photomicrograph of arkosic sandstone showing glauconite that has replaced biotite 68 



54. View northeastward from Bald Mountain showing outcrops of Temblor formation 72 



55. Topographic map and section of landslide in the New Almaden mine area 77 



56. Map showing distribution of Calera-type limestone and altered tachylitic greenstone 80 



57. Map showing structural blocks in the New Almaden district and the major faults in them 81 



58. Map showing the major structural features in the older rocks 83 



59. Plastically deformed and sheared rocks of the Franciscan group 86 



60. Interbedded graywacke and siltstone showing typical irregularities due to rock flowage and minor faulting... 87 



61. Plastically deformed and faulted layer of greenstone 87 



62. Map showing the major structural features in the post-Franciscan rocks 89 



63. Fairly regular contact between serpentine and alta 94 



64. Irregular contact between serpentine and alta 94 



65. Photomicrograph of section across banded dolomite vein containing cinnabar, pyrite, and hematite 98 



66. Stibnite in dolomite vein cutting earlier dolomite vein 99 



67. Silica-carbonate rock cut by typical hilo 100 



68. Postore dolomite vein 101 



69. Photomicrographs of "froth vein" formed by deposition from immiscible fluids 102 



70. Polished specimen of rock ore from the New Almaden mine 103 



71. Large polished slab of ore from the New Almaden mine 104 



72. Polished surface on ore specimen from Randol workings of the New Almaden mine 105 



73. Polished surface on rich ore formed by replacement of silica-carbonate rock, which, in turn, had replaced 



sheared serpentine 106 



74. Extremely rich replacement ore from the New Inclined shaft area of the Guadalupe mine 108 



75. Key to figure 74 1 09 



76. Photomicrograph of rich ore formed by replacement of silica-carbonate rock by cinnabar 110 



77 Photomicrograph of high-grade ore formed by replacement of silica-carbonate rock by cinnabar 110 



78. Map of La Ventura stope of the New Almaden mine 111 



79. Map of the New Ardilla stope of the New Almaden mine showing the localization of ore along a swarm of hilos, 112 



80. Map of a part of the 600 level of the Harry workings of the New Almaden mine showing an area of unmin- 



eralized silica-carbonate rock containing swarms of hilos 113 



81 . Map show distribution of stopes along the upper surface of the main serpentine sill in the New Almaden mine . 114 



82. Map showing the localization of the Machine stope ore body of the New Almaden mine beneath a small 



dome on the upper contact of an intrusive sill 115 



83. Map showing the localization of the New World stope ore body of the New Almaden mine beneath a plunging 



inverted trough on the surface of an intrusive sill 116 



84. Cross section through the Giant Powder-Ponce stope area of the New Almaden mine showing the localization 



of ore in an anticlinal warp on the upper surface of a sill of serpentine 116 



