72 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIODRE 54. View northeastward from Bald Mountain. Banded aspect of hill In center Is due to outcrops of more resistant sandstone beds 

 of the Temblor formation. Crest beyond Is a part of Los Capltandlloa Ridge ; America mine Is on left and opencuts of New Abnaden 

 mine on right. 



sandstone. The arkosic sediments of the lower part 

 are best exposed along a road that extends between 

 Guadalupe and Almaden Canyons across the long 

 ridge lying between Bald Mountain and Mine Hill. 

 The roadcuts through a stratigraphic sequence at 

 least 2,150 feet thick of predominantly massive and 

 coarse-grained, white or light-buff, well-sorted sand- 

 stone, mostly in beds from 12 to 30 inches in thickness. 

 Sequences of several resistant beds as much as 20 feet 

 thick in aggregate thickness are exposed almost con- 

 tinuously along their strike in white, moss-covered out- 

 crops, which give the hill the banded appearance 

 shown in figure 54. The sandstone are all well ce- 

 mented with calcite, and many beds contain numerous 

 fragments of mollusca shells, which in some places are 

 so concentrated as to form beds of organic limestone. 

 Most of the fragments are of thick-shelled oysters, 

 but two fairly complete pelecypods from the sandstone 

 have been identified by Miss Myra Keen, of Stanford 

 University, as Mitilis expansus Arnold(?) and M. 

 loeli Grant. 



Intercalated with the sandstone are beds of conglom- 

 erate having a matrix similar to the sandstone in grain 

 size and mineral composition. The pebbles and larger 

 blocks in the conglomerate consist principally of a 



typical assemblage of the older rocks found on the 

 slopes of the Sierra Azul. In order of apparent abun- 

 dance are pebbles of graywacke, chert, shale, amphib- 

 olite, serpentine, and greenstone. The pebbles are di- 

 visible on the basis of their origin into two classes. 

 One class consists largely of typical rocks of the 

 Franciscan group, together with some serpentine boul- 

 ders which are peculiar in that their outer surfaces 

 are weathered and stained a deep red. All sizes be- 

 tween pebbles less than half an inch in diameter and 

 boulders as much as 2 feet in diameier arc common; 

 the shapes range from rounded to angular. The other 

 class of pebbles consists of hard rocks, dominant ly 

 black chert, white vein quart/., and quart/, porphyry; 

 these are well rounded and very smooth, some having 

 a polished surface. Some pebbles are more than 4 

 inches in diameter, but most are less than 1 inch. They 

 are probably reworked pebbles from older conglomer- 

 ate beds, such as the conglomerate of the Sierra Azul. 

 Similar smooth fragments of hard rock are found in 

 much of the sandstone in the lower Temblor, though 

 in these rocks they are generally in smaller grains. 



Stnitigniphically, about 1,500 feet above the oldest 

 exposed coarse-grained sandstone and conglomerate 

 beds are layers of medium-grained buff sandstone of 



