18 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIGURE 7. Photomicrograph of siltatone of the Franciscan group. 

 Note the abundance of angular fragments, largely plagioclase and 

 quartz, and the relatively small amount of clay or fine-grained 

 matrix material. Bedding rung horizontally across photomicro- 

 graph. 



rarely exposed at the surface. Very locally in the 

 siltstone, small elliptical concretions of limestone 6 to 

 8 inches in diameter are developed. A few of these 

 concretions found in a thin-bedded dark rock exposed 

 at the base of the Calero Dam proved to be of un- 

 usual interest because they contained fragments of 

 marine megafossils. 



Microscopic features 



The small amount of microscopic work done on these 

 fine-grained rocks shows that they have approximately 

 the same mineral content as the graywacke, except 

 that they contain a little more mica, clay, and car- 

 bonaceous matter (fig. 7). Wood fragments large 

 enough to distinguish with a hand lens are fairly com- 

 mon in the Franciscan siltstones elsewhere in the 

 Coast Ranges, but not in those of the New Alinaden 

 district. 



Chemical features 



An analysis of black siltstone of the Franciscan 

 group from Fern Peak (formerly called Fern Hill) 

 in the New Almnden district is given in table 2, col- 

 umn 1. As compared with the analysis of graywaeke 

 shown in the next column, the siltstone contains less 

 silica and a little more aluminum, iron, and magne- 

 sium. It also contains a little less ralrinni. even when 

 the calcium necessary to form calcite from the carbon 

 dioxide is excluded. These differences indicate that 

 the siltstone contains more fragments of mafic rock 

 and a little more clay than the analyzed graywacke. 

 A comparison of the siltstone of the Franciscan group 



with average shales may be made by referring to table 

 2, column 3, which gives a composite of 78 analyses 

 of shales. The analyzed rock from the New Almaden 

 district contains more silica, magnesium, and sodium 

 and less aluminum, ferric iron, calcium, potassium, 

 and combined water than the average shale. It also 

 has a potash-soda ratio of less than 1. These differ- 

 ences are those that would be expected from the low 

 clay content of the siltstones of the Franciscan group 

 and the soda-rich character of all the clastic sedimen- 

 tary rocks of the group. 



TABLE 2. Analysis of sillstone of the Franciscan group, with 

 analyses of graywacke and a composite of 78 shales for com- 

 parison. 



NOTE. Description of sample and locality as follows: 



1. Black siltstone (NA-31S) from Fem Peak, New Almaden district, Santa Clara 



County, Calif. Mrs. A. C. Vlisklis, U.S. Geological Survey, analyst. 



2. Oraywacke, see table 1, this report. 



3. Composite of 78 shales from Clarke, 1924, p. 831. SO) recast as S; BaO omitted 



ALTA 



A distinctive variety of rock composed largely of 

 sheared shale envelopes the serpentine masses. Many 

 of the quicksilver deposits in the Coast Ranges are 

 closely associated with this rock, which has become 

 widely known to California quicksilver miners as aha. 

 It received this Spanish name, meaning "hanging 

 wall," from the Mexican miners in the early Jays. 

 because it commonly overlay the ore bodies that were 

 found along the upper margins of altered serpentine 

 sills. The alta, however, has been found to be just a- 

 common along the lower sides of the serpentine sills, 

 where in places it forms the footwall for ore bodies. 

 Similar rock also occurs along fault /.ones that traverse 

 rocks of the Franciscan group. 



The alta, like a fault breccia or mylonito, owes its 

 character as much to shearing as to original lithology. 

 but siltstone or shale is everywhere its most abundant 



