48 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



rounded blocks of unsheared serpentine in a completely 

 sheared matrix (fig. 35). The proportion of matrix 

 to blocks, as seen in artificial cuts where exposures are 

 perfect, varies within wide limits: the sheared matrix 

 may form only thin separations between massive blocks 

 that nearly touch one another, or it may be relatively 

 abundant, containing only here and there a small 

 rounded pod of unsheared serpentine. All gradations 

 in the relative amounts of sheared matrix and un- 

 sheared blocks may be found. The blocks, where 

 fresh, are dark green, nearly black, in color and have 

 a pseudoporphyritic texture wherein ragged crystals 

 of bastite a quarter of an inch long, derived from py- 

 roxene, are scattered through a matrix of deep-green 

 granular serpentine derived from olivine. Many 

 blocks also contain magnetite, either disseminated as 

 individual crystals a little less than 1 mm across, or 

 concentrated in veinlets. 



The field appearance of the serpentine masses de- 

 pends on how much they are sheared and on whether 

 they crop out in the low foothills or in the higher 

 mountains. The large masses are generally blocky and 

 can be distinguished as serpentine from a distance. In 



the lower foothills these give rise to distinctive green- 

 ish or drab-colored slopes studded with groups and 

 trains of closely spaced boulders as much as 20 feet 

 in diameter (fig. 36). From a distance many of these 

 slopes display a crude banding, caused by alternation 

 of bands of large and small boulders, or by a succes- 

 sion of more sheared and less sheared zones. This 

 banding generally is nearly parallel to the contacts of 

 the serpentine mass, but in parts of the large and ex- 

 ceptionally well-exposed mass in the Santa Teresa 

 Hills it can be seen to diverge from the contacts MI 

 angles as great as 30. 



The margins of the masses in the foothills are 

 sharply marked in many places by slight topographic 

 bulges in the peripheral serpentine and a shallow, 

 but perceptible, flattening of the slopes just below 

 (fig. 37). This topographic expression tends to be 

 obscured, however, by small landslides, which are 

 common along these contacts and give rise to small 

 seeps or springs. In some areas in which the ser- 

 pentine is well exposed, linear grass-covered patches 

 devoid of boulders are conspicuous; these patches are 

 generally underlain by septa of sedimentary or vol- 



FIOITRB 36. Blocky serpentine exposed In fresh rondcnt. The blocks are Irr.-unlur In shape, hut have sllokenslded Rurfaces and 

 rounded corners and edges. They Invariably show undisturtecl relict textures Inherited from the original perldotlte or dunlte. 

 Between the blocks the serpentine Is Intensely sheared and does not exhibit any relict textures. 



