Gold 



309 



Nevada uplift, fall into two groups in point of age. The east- 

 ern half of the belt, and higher up on the Sierra slope, is much 

 older than the western and lower half. This has been deter- 

 mined from a few fossils that have been found, chiefly in the 

 limestones and sandstones. From these it has been established 

 that the eastern portion of the belt, that higher up the slope, 

 is of Palaeozoic (Carboniferous) age, the Calaveras formation. 

 The rocks of the western and lower portion of the belt are of 

 Mesozoic (Jurassic) age, the Mariposa formation. It is thought 

 that formations representing all the periods of Palaeozoic time 

 may be included in the group of Calaveras rocks. 



Courtesy U. S. Bureau of Mines 



FIG. 88. Barite ore, from National Lead Company's mine near El Portal, 

 showing intense folding (actual size). 



"The rocks of all the formations, the Calaveras and the Mari- 

 posa, together with intercalated lavas which were poured out 

 during the time of formation of the latter, all are intensely 

 folded and compressed, and are much alike in general aspect. 

 However, they belong to two different systems. In the greatly 

 deformed structure of these two distinct series of strata there 

 is clear evidence of the former existence of two successive 



