GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



iron oxides. The sandstone is homogeneous, moder- 

 ately well sorted, and medium to coarse grained. Most 

 of the grains are subangular, a few are angular, and 

 others are subrounded. Their average diameter is a 

 little less than 0.5 mm, and nearly all the diameters 

 fall within a range of 0.2 to 1.0 mm. Rounded grains 

 of chert are found in some specimens; these are gener- 

 ally a little larger than the other grains, some of them 

 being as much as 3 mm in diameter. Cementing mate- 

 rial is present only in small amount and appears to 

 consist of clay and limonite. Judging from a study 

 of only a few thin sections of the rocks, they contain 

 from 50 to 75 percent of quartz; up to 30 percent of 

 orthoclase, myrmekite, and oligoclase; a few percent 

 of microcline; and a little biotite, muscovite, glau- 

 conite, sphene, magnetite, and cloudy limonite. Rock 

 fragments also are present but are not abundant. One 

 variety of the sandstone, occurring north of the mouth 

 of Almaden Canyon and also in one of the fault slivers 

 north of the Senator mine, weathers to a rock with a 

 striking chocolate-brown porous peripheral zone that 

 is sharply separated from a core of umveathered gray 

 fine-grained sandstone. In thin section this sandstone 

 is seen to have a calcite cement and to include more 

 than 5 percent of brown biotite which is largely al- 

 tered through a green biotite stage to glauconite (Gal- 

 liher, 1935, p. 1351-1365). (See fig. 53.) 



The shales of Late Cretaceous age in the Santa 

 Teresa Hills are not exposed, but they are believed 

 to underlie several areas of rolling grasslands that 

 have a sticky deep-brown soil containing sparse frag- 

 ments of dark shale and scattered limy concretions. 

 In one of the fault slivers north of the Senator mine, 

 shale is exposed in a sharply incised canyon. In this 

 small area at least, the shale is dark greenish gray and 

 thin bedded, and breaks with a hackly or curved frac- 

 ture. It resembles some of the shale and siltstone of 

 the Franciscan group; however, it is more distinctly 

 bedded and more clayey, and lacks the sheen charac- 

 teristic of cleavage surface of the shales of the Fran- 

 ciscan group. At this locality, and also in the lowest 

 shale beds in the Santa Teresa Hills, limy concretions 

 averaging about 1 foot in diameter are characteristic. 

 Two varieties of these concretions are common. One 

 variety is generally dark brown and septarian. and 

 has crack fillings of yellowish calcite: some of these 

 concretions contain a few fossil fragments. The other 

 variety is chalky on the surface and white or buff on 

 the inside. These concretions contain minute spherical 

 transparent bodies, which probably represent organi<- 

 remains that are too poorly preserved to be identified. 



Fossils arc exceeding! V rare ill the I pper ( ret arenus 



' <>f a fos - 



FIGURE 52. Igloollke rock ID the Upper Cretaceous sandstone of the 

 Santa Teresa Hills resulting from spheroidal and cavernous weather- 

 Ing. The lower surfaces of the caverns are flat and act as a collect- 

 ing basin for rainwater, which, on standing, dissolves the cementing 

 material from the rock beneath. When dry, the loosened sand is 

 apparently removed by wind or animals. 



on the gentle dip slopes, to flat-bottomed caves and 

 pits that are found in the bouldery area. An extreme 

 type of weathering, resulting from the hollowing of 

 a spheroidal boulder, is the igloolike rock shown in 

 figure 52. 



Lithologically the Upper Cretaceous sandstone of 

 the Santa Teresa Hills is fairly uniform throughout 

 its extent. Where fresh it is light gray in color, but 

 the more common weathered sandstone is buff colored. 

 Some outcrops have a uniform reddish tinge, and some 

 exhibit concentric or wavy bands of red and brown 



Fiona* 53. Photomicrograph of arkoslc sandstone of Late Oretaeeou. 

 age from north of the Senator mine, showing glauconite that has 



'' orthocla " <0 



