50 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



natural exposures of the serpentine. In the higher 

 country the sheared serpentine, like the blocky vari- 

 ety, supports a growth of manzanita, and in some 

 places at lower altitudes tarweed is particularly abun- 

 dant on it. During a few days in the early summer 

 it is particularly easy to outline the masses of sheared 

 serpentine on a soil covered slope, for the grass in the 

 soil derived from serpentine remains green a little 

 longer than it does in the soils derived from other 

 rocks. By making use of these criteria, generally one 

 may locate and crudely outline masses of sheared ser- 

 pentine rather quickly, but determining their exact 

 outlines is difficult and cannot everywhere be done 

 with certainty. 



Where the sheared serpentine is silicified, whether 

 in larger masses or in septa between boulders of un- 

 sheared serpentine, it forms on weathering jagged, 

 spired, and crudely tabular angular outcrops, which 

 show at a glance the prevalent direction of shearing. 

 In a few outcrops of silicified sheared serpentine 

 snow-white puff balls of magnesite, generally a little 

 less than 1 inch in diameter, are conspicuous; these 

 balls of magnesite, however, being resistant to weath- 

 ering, commonly fall out, leaving a pock-marked sur- 

 face. 



Massive serpentine derived from dunite containing 



no pyroxenes is rare in the area, but it was found in 

 several small scattered exposures. It can be distin- 

 guished only where it is unsheared, but there it is 

 readily recognized in both fresh and weathered expo- 

 sures by its granular texture, lack of bastitic pseudo- 

 morphs, and typical irregular veining by various ser- 

 pentine minerals (fig. 38). Where it is slightly 

 weathered, it assumes a light-gray, nearly white color 

 and is not unlike a fine-grained sandstone; examina- 

 tion with a hand lens, however, invariably reveals the 

 presence of minute black crystals of picotite, easily 

 recognized by their submetallic subadamantine luster. 

 In most places, however, the serpentine derived from 

 dunite is more intensely weathered, and part of the 

 rock has been hardened through a width of about a 

 quarter of an inch along many irregular veinlets and 

 fractures; the hardened zones are deep green and re- 

 sistant, and the intervening serpentine is whitened and 

 leached out, so that the surface is full of highly irreg- 

 ular deep cavities from 1/4 to 1 inch in diameter. 



Microscopic features 



As the serpentine minerals are variable in optical 

 properties and variously grouped under different 

 names, it, is appropriate to discuss the nomenclature 

 used in this report before describing the appearance 

 of the serpentine in thin section. A survey of the 



1 inch 



88. Pollihed surface on fresh serpentine derived from dunite. The latest veins are a waxy porcelaneous serpentine 

 that replace* older serpentine minerals. This porcelaneous material Is apparently what was originally referred to by 

 Lodochnlkor as "serpopbtte." (See p. BO.) 



