74 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



3. There is no evidence of baking either above or 



below. 



4. It is everywhere apparently conformable with the 



other sediments. 



The most prominent mass of Miocene volcanic rocks 

 forms Lone Hill an elliptical knoll rising 120 feet 

 above the floor of the Santa Clara Valley about 4 

 miles northeast of Los Gatos. The hill has been re- 

 garded as a volcanic neck, but it is merely an erosional 

 remnant of eruptive rock. It is underlain by three 

 varieties of volcanic rock which cross the hill in ir- 

 regular layers that strike slightly west of north and 

 dip, in general, steeply eastward. The southwestern 

 base of Lone Hill and a rise just south of it are under 

 lain by altered well-bedded whitish rocks that are re- 

 garded as pyroclastic tuffs, though they differ little 

 from the more tuffaceous varieties of shale in the 

 Monterey. Lying unconformably on the pyroclastic 

 rocks is a layer of perlitic dacite, which in some pi 

 is as much as 200 feet thick. Where fresh, the perlitic 

 rock is light gray and exhibits a glassy matrix flecked 

 with plagioclase phenocrysts, a few quartz crystals. 

 and deformed cavities filled with secondary minerals. 

 The plagioclase has cores of andesine and rims of 

 albite-oligoclase. Stratigraphically above the perlite 

 and underlying the greater part of the hill is a body 

 of massive and flinty gray dacite. It is exposed in 

 rather small blocky weathered outcrops that break 

 along irregular, somewhat conchoidal fractures. Some 

 of it is vesicular, and some exhibits irregular flow 

 banding. The dacite contains varying proportions of 

 small phenocrysts of quartz and glassy andesine, which 

 are generally unoriented, and slightly larger dull- 

 white fine-grained fragments that are probably tuff. 

 Ferromagnesian minerals were originally present in 

 small amount, but are completely replaced ; the shapes 

 of pseudomorphs indicate that the rock contained both 

 a pyroxene and an amphibole. All the various kinds 

 of volcanic rocks on Lone Hill are in most places ex- 

 tensively altered. They are in large part kaolini/.ed 

 and pyritized, and much of the more porous rock con- 

 tains high-temperature forms of silica. Both cristo- 

 balite and tridymite occur so abundantly in the 

 groundmass and in cavities as to have attracted the 

 attention of mineralogists and mineral collectors. 



The other smaller bodies of dacite shown on the 

 map (pi. 1) are composed of similar rocks all greatly 

 altered. Their occurrence along faults and as isolated 

 bodies suggests that they are intrusive, but most of 

 them are so brecciated and silicified that their original 

 character is obliterated. The dacite that projects 

 through the alluvium at tho oast end of the hill <>f 

 Eocene rocks north of the Senator mine is of special 



interest, because it locally contains cinnabar in suffi- 

 cient quantity to be readily visible without the aid of 

 a hand lens. 



MonU'ri'.v slmli- 



The sedimentary rocks of the Monterey in the dis- 

 trict conformably overlie the sedimentary and volcanic 

 rocks of the Temblor formation. They consist largely 

 of extremely fine grained dazzling-white siliceous rick> 

 which are mostly diatomaceous. but near the base of 

 the formation these are interbedded with fine- to 

 medium-grained light-gray sandstone and massive 

 white sandstone. The exposed thickness of the for- 

 mation is 1,300 feet, but its top is nowhere expo 

 The exact time range of the sedimentary rocks doubt - 

 less could be ascertained from their foraminiferal con- 

 tent, but such a study has not been made: the age of 

 the formation is regarded as late Miocene only because 

 most of the diatomaceous shale in the Coast Ranges 

 was deposited at this time. 



Between the Temblor and Monterey formations in 

 part of the area is a transitional /.one about 100 feet 

 thick in which porcelanite and diatomaceous shale are 

 intercalated with bentonitic shale, coarse-grained buff 

 sandstone, and reddish conglomerate with pebbles up 

 to 4 inches in diameter. The rocks of this /.one gen- 

 erally do not crop out, but rather disintegrate to form 

 a pale ocher-colored sandy soil containing diatomite 

 fragments and rounded pebbles. They are well ex- 

 posed at only one place in a cut on a road branching 

 south from Shannon Road, about 2 miles east of Los 

 Gatos. The bentonite exposed here is a homogeneous 

 greenish-gray plastic clay occurring in beds up to !>> 

 inches thick. Under the microscope no fragments] 

 material or shards were found in the bentonite. but 

 from its refractive index it is believed to be largely 

 montmorillonite. The buff sandy layers average about 

 1 foot in thickness and are poorly consolidated: they 

 contain conspicuous, though not abundant, small well- 

 rounded pebbles of black chert. The conglomerate 

 beds, also poorly consolidated, consist predominantly 

 of cobbles of dark-brown medium-grained sandstone 

 and blocky fragments of indurated shale. In other 

 parts of the area the contact bet \veen the Temblor 

 and Monterey formations coincides with the top of 

 the tuff-breccia, and in still other places there is no 

 gradation between arenaceous and siliceous sedimen- 

 tary rocks. 



The typical rocks of the Monterey are most exten- 

 sive in the Blossom Hill area east of Los (iatos. where 

 they underlie, steep-sided rounded hills that are ex- 

 tensively planted in vineyards and fruit orchards. Al- 

 though the siliceous shales are well exposed in many 

 artificial cuts, they form few natural outcrops, because 



