SILICA-CARBONATE ROCK 



59 



FIGURE 41. Silica-carbonate rock derived from sheared serpentine. The white veinlet is dolomite. This specimen Is typical 

 of the rock that is the host for nearly all the quicksilver ore bodies in the district. 



lar or streaked-out appearance, although it shows very 

 little tendency to break parallel to the shear planes 

 inherited from the serpentine. (See fig. 41.) Lo- 

 cally, it may show textures interpreted as resulting 

 from complete replacement of chrysotile veinlets or 

 bastite pseudomorphs, but more commonly these tex- 

 tures have been removed by shearing before the altera- 

 tion of the serpentine. The different lenticules and 

 streaks are generally gray and green of various shades, 

 and the sheared texture is accentuated by veining with 

 light-colored dolomite or quartz. Hence the overall 

 color of most of the silica-carbonate rock is green or 

 greenish gray, but an unusually siliceous kind found 

 only west of the Guadalupe mine is almost black. The 

 hardness and mode of fracturing of the rock depend 

 upon the proportion of silica to carbonate, and in a 



less degree upon their grain size and distribution. 

 Carbonate-rich rock resembles fine-grained marble, be- 

 ing fairly soft and having an irregular fracture, 

 whereas rock rich in silica is more like chert, being 

 hard and having a relatively smooth conchoidal frac- 

 ture. The more common intermediate varieties are in 

 most places surprisingly hard and tough, but they 

 break with a rough fracture. 



Weathering generally makes a radical change in 

 the appearance of the silica-carbonate rock, because 

 the ferroan magnesite is removed by weathering, leav- 

 ing only hydrated ferric oxides and silica. All de- 

 grees of exposure are seen in the district. In a few 

 places, where the weathered rocks contained little 

 silica, they do not crop out at all, but instead give 

 rise to an ocherous soil containing only a few siliceous 



