14 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIGURE 4. Deformed graywacke exposed In roadcut between Bald Mountain and the crest of the Sierra Ami. The Irregular folds and low- 

 angle thrust I'M ul is, such as are shown here, are abundant throughout much of the district, but In many places sufficient shale accompanies 

 the graywacke to permit rock flowage to predominate overslmple folding and faulting. 



that have been plastically deformed so that their 

 present thicknesses are quite different from their 

 original thicknesses. 



Original variation in the thickness of beds of 

 graywacke, or the accompanying beds of shale or 

 tuff, however, are the general rule, and no systematic 

 relation between the thickness of the graywacke and 

 other interbedded rock was observed. Most ex- 

 posures show a completely erratic distribution of beds 

 of different thicknesses, although locally there are 

 sequences of thick or thin beds. The maximum thick- 

 ness of beds seen in artificial exposures rarely exceeds 

 5 feet, though surface outcrops several times as wide 

 without apparent bedding are not uncommon. 

 Sharply limited thin layers of graywacke and shale 

 also can be seen in some places. Lenticular beds were 

 observed, especially where thin layers of graywacke 

 are interbedded with shale or tuff, but more com- 

 monly slippage along breaks that are nearly parallel 

 to bedding has formed lenticles that do not neces- 

 sarily reflect original lenticular bedding. Graded 



bedding, with the graywacke grading upward to shale 

 by an imperceptible decrease in grain size, accom- 

 panied by a darkening of color, is present in some of 

 the sedimentary rocks but cannot be seen in most 

 places. Current bedding on a small scale was ob- 

 served in thin-bedded sedimentary rocks in some ex- 

 posures in the mines, but apparently it is rare as it 

 was not observed in the poorer surface exposures. 

 Flutings were noted on the lower surfaces of the 

 graywacke layers, but they are rarely found. None 

 of the sedimentary rocks contain ripple marks. Vein- 

 lets of either quartz or calcite, or both, are abundant, 

 and the weathered surfaces of rocks partly replaced 

 by calcite are covered with small irregular pits. 



The lithic graywackes that are most abundant in 

 the central part of the district contain a high pro- 

 portion of malic rock fragments and seem to trrade 

 into tufts and breccias mapped as greenstone. They 

 also contain in places erratically distributed green- 

 stone pebbles, or even boulders of greenstone several 

 feet in diameter. 



