FUTURE PRODUCTION 



173 



prevalent during the development of the mine that 

 are now believed to be unsound. 



The first of these unsound beliefs was that the ore 

 almost everywhere lay along the upper side of a body 

 of silica-carbonate rock and beneath the alta, the very 

 name of which implies its supposed position above the 

 ore. This belief had some foundation in that the 

 majority of the ore bodies found were so situated 

 although one reason for this was that search for new 

 bodies was concentrated beneath the alta. But since 

 many other ore bodies that lay above the alta and on 

 the lower sides of silica-carbonate rock masses were 

 found unexpectedly, as described on page 115, this 

 notion should not have been so closely followed in the 

 development of the mine. 



The second belief was that the serpentine, along the 

 borders of which ore bodies were found, formed a 

 stocklike intrusive mass, which though highly irregu- 

 lar widened downward. This idea is well illustrated 

 in figure 1004, and for comparison our concept of two 

 north-dipping convergent sills is indicated in figure 

 1005, which is a generalized cross section drawn along 

 the same line as figure 1004. 



In order to appraise the possibilities for finding 

 new ore bodies in the mine that become apparent 

 when these two beliefs are rejected, let us consider 

 the contacts numbered 1 to 4 in figure 1005. along all 

 of which there may remain unexplored places where 

 ore bodies may be discovered. Along contact 1, large 

 ore bodies were found in former years, including the 

 great Santa Rita, the North and South Randol, Velasco, 

 Harry, and Cora Blanca ore bodies. This contact, 

 which according to the belief held during the devel- 

 opment of the mine is the northern and eastern border 

 of the stocklike mass, was far more thoroughly ex- 

 plored than any of the other three, but a few parts of 

 it still seem worthy of further exploration. Contact 

 2 is explored in places between the 900 and 500 levels, 

 and just above it in this interval the El Collegio, 

 Santa Rosa, Buenos Ayres, Far West, Sacramento, 

 and New Ardilla ore bodies were found (pi. 4). This 

 contact was also penetrated near the 1800 level by the 

 lower part of the Randol shaft and the 1800-level 

 crosscut driven northward from it. (See section 

 B-B' pi. 11.) Contact 3 is partly explored between 

 the 800 and 500 levels, and in. this interval some ore 

 was found adjacent to it. It was also reached by a 

 crosscut driven south from the Randol shaft on the 

 1800 level. Contact 4 is unexplored except where it 

 was crossed on the 900 level, and followed on the 1000 

 level, in the southwestern part of the San Francisco 

 area. The existence of the contact beneath the Day 

 tunnel is largely inferred from the arch of silica- 



carbonate rock penetrated by the tunnel and by the 

 fact that exploration in the lower levels of the San 

 Francisco area extended back under the serpentine 

 body without encountering it. The evidence afforded 

 by the silica-carbonate rock body becomes increasingly 

 strong when considered in connection with the fact 

 that in the New Almaden mine area silica-carbonate 

 rock is almost invariably found only along the mar- 

 gins of large serpentine bodies and not within them. 



Contact 1 



The shape of contact 1 and the positions of the 

 stopes lying just beneath it are shown in figure 81. 

 Unexplored parts of the contact are indicated by dot- 

 ted contours where their approximate position is 

 known, but between the Cora Blanca area and the 

 isolated San Francisco area in the southern part of 

 the area mapped, where the contact is believed to ex- 

 tend across an arch, even dotted contours have been 

 omitted owing to the complete absence of data indi- 

 cating its position. Places believed to deserve fur- 

 ther attention are the gently plunging arch east of 

 the Santa Maria shaft; the relatively flat part of the 

 contact between the Santa Rita, Velasco, and Harry 

 stopes; and the possible arch between the Cora Blanca 

 and San Francisco areas. 



The first of these places is thought to be particu- 

 larly promising for several reasons. The contact forms 

 a gently plunging arch, a favorable structure for the 

 localization of ore bodies. Small quantities of cinna- 

 bar are reported to have been found in silica-carbonate 

 rock at the north end of the 800 level (represented by 

 the 1000-foot contour line) in the Harry area, and 

 hilos are abundant in the northern parts of both the 

 700 and 800 levels in the same area. Ore solutions 

 are known to have passed through this area, as thin 

 films of cinnabar were found in the chert of the Fran- 

 ciscan group near the summit of Church Hill. This 

 favorable structure could be tested most easily by ex- 

 ploratory drilling from the surface to depths of only 

 a few hundred feet along a zone extending from the 

 main camp to the top of Church Hill. It could also 

 be explored underground by drifting on the 800 level : 

 the contact could be followed eastward from the Day 

 tunnel and raises put up in places where hilos were 

 found to be abundant. 



The second favorable area on contact 1 lies between 

 the Santa Rita, Velasco, and Harry stopes, at an eleva- 

 tion of 1,200 to 1,300 feet. This area has been pene- 

 trated only by 2 raises from the Tonkin crosscut on 

 the 700 level. The southern raise penetrated silica- 

 carbonate rock containing some cinnabar; the north- 



