70 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



hills near the mouth of Guadalupe Canyon. Most of 

 the sandstone lies stratigraphically above the shale, 

 but the lowest beds seem to grade into shale along the 

 strike towards the east. The outcrops of the sandstone 

 are marked by concentrations of weathered low-lying 

 rounded light-buff or light-gray boulders, the largest 

 more than 5 feet in greatest diameter, and in most 

 places the sandstone shows only a trace of bedding. 

 Accumulation of iron oxides on and near the weath- 

 ered surfaces results in casehardening, and beneath the 

 surface it gives rise to brownish-red and pinkish 

 blotches and bands. Iron oxide also imparts a pink- 

 ish-brown color to a distinctive sandy soil that forms 

 on the sandstone. In parts of the area this soil sup- 

 ports thick growths of brush which contrast with the 

 grassy areas underlain by shale. 



The typical unweatherecl sandstone is light gray, 

 coarse-grained, and poorly sorted. It contains angular 

 clasts of glassy quartz, feldspar, and rock fragments 

 cemented by interstitial fine-grained calcite. In the 

 weathered rocks, the close-packed clasts are in contact, 

 and the interstitial calcite has been leached away pro- 

 ducing a porous texture. The clasts in some places 

 are as much as 1.5 mm in maximum diameter, but they 

 average only a little more than 0.5 mm. Quartz is 

 relatively more abundant in the sandstone of Eocene 

 age than in that of the Upper Cretaceous rocks of the 

 Santa Teresa Hills, though sodic plagioclase, myrmek- 

 ite, microcline, and orthoclase together may make up 

 as much as 20 percent of the rock. In some places the 

 feldspar has been partly altered to clay minerals, but 

 elsewhere it is fresh. Detrital sericite, hornblende, 

 and epidote occur in very small amount. Scattered 

 throughout the rock are shale fragments that are com- 

 monly rimmed with stains of iron oxides. 



A special phase of the sandstone, commonly occur- 

 ring near lenses of limestone and referred to in the 

 field as the "white sandstone," resembles the "glass 

 sand" of the Meganos formation north of Mount 

 Diablo. It contains about 20 percent of finely crys- 

 talline calcite cement, which is more than is found 

 in the typical sandstone of the formation, and on 

 weathered surfaces the grains stand out in stronger 

 relief. Thin sections show large clastic grains of 

 quartz and feldspar, surrounded by a calcite matrix 

 which contains some quartz and feldspar fragments 

 less than 0.1 mm in average diameter. Rocks repre- 

 senting all gradations between this "white sandstone" 

 and sandy limestone may be observed, and in many 

 places the sandstone contains echinoid spines, broken 

 shells, and a few large Foraminifera. 



Several lenses of limestone are interbedded with the 



shale near the base of the unit in the Santa Teresa 

 Hills. The thickest of these, exposed in a quarry near 

 the Bernal mine, measures about ~2~> feet in thickness, 

 but most are only 3 to 5 feet thick. Some of the 

 lenses are several hundred feet long, but many are so 

 small that each is indicated only by a few boulders. 

 As these lenses contain several different kinds of lime- 

 stone, their exposures differ in character. In general. 

 however, the limestone is more resistant to erosion 

 than the surrounding rocks, and its outcrops are 

 marked by rounded boulders, by trains of low boul- 

 ders, or more rarely by inclined rounded tablets flat- 

 tened parallel to the bedding. The different kinds of 

 limestone tend to grade into one another, but they 

 may be divided for description into three varieties. 



One variety of limestone, best exposed in the quarry 

 near the Bernal mine, is flinty, generally massive, and 

 poorly bedded. It weathers in such a way that the 

 surface of its bouldery outcrops, and even most of the 

 quarry faces, are encrusted with a dazzling-white tine- 

 grained chalk: but fresh exposures of the rock are 

 commonly mottled or locally handed in shades of tan 

 or light gray. In places it is blotchy, containing ir- 

 regular masses of medium-gray coarsely crystalline 

 limestone, which, when freshly broken, has a faint 

 odor of crude petroleum. Veinlets of clear coarsely 

 crystalline calcite cut all the rock, and it contains nu- 

 merous vugs, some of them as much as 1 inch in diam- 

 eter, which are lined with crystals of calcite. In gen- 

 eral, this variety of limestone is not very fossil if erous, 

 but it does contain some small gastropods and pelecy 

 pods, the most abundant of which is Pltnr cf /'. C<iH- 

 fornianus (Conrad). 



A second variety of limestone occurs interbedded 

 with the coarse-grained sandstone in the Santa Teresa 

 Hills and appears to be stratigraphically higher than 

 the limestone exposed in the quarry. It is generally 

 characterized by a fragment al appearance, but its tex- 

 tural and mineralogic makeup differs from place to 

 place. The rock appears to have been formed by mix- 

 ing of coarse arenaceous material with highly fossilif- 

 erous calcareous material, for it is made up of lenses 

 and irregularly inlet-fingered masses of sandstone, fine 

 conglomerate, and clastic limestone. The clastic lime- 

 stone contains irregularly broken fragments of gas- 

 tropods, pelecypods, Foraminifera. and reef-forming 

 organisms, such as coralline algae and bryo/oans, to- 

 gether with rounded chert pebbles and fragments of 

 other rocks. T. Wayland Yaughan. of the U.S. Geo- 

 logical Survey, examined a small collection of fossils 

 from three different limestone masses in the Santa 

 Teresa Hills ami identified Axti-riH-i/cliiw aster 



