292 



University of California Publications. [Geology 



THE PALAEOZOIC ROCKS. 



The sedimentary rocks of the Robinson Mining District are 

 confined to the Palaeozoic. The npper portion of the section has 

 been determined on palaeontological evidence to be of Carbon- 

 iferous age. In the study of the structure of the district it has 

 been found convenient to subdivide this Carboniferous series 

 into three formations and for these local names are used. The 

 lower part of the section has yielded no fossils, but falls into 

 two strongly contrasted members, and these, on the basis petro- 

 graphic and other considerations to be mentioned later, are cor- 

 related with the two divisions of the Devonian recognized else- 

 where in the basin ranges, viz : the White Pine shale and the 

 Nevada limestone, and they will therefore be described under 

 these names. 



THE CARBONIFEROUS. 



The Ruth Limestone. — The highest member of the Carbonif- 

 erous series is the Ruth limestone, a formation of heavy, but 

 distinctly bedded, blue gray limestone, carrying Upper Carbon- 

 iferous fossils. This formation is characterized by an abundance 

 of flint or chert nodules and the fossils are to a large extent 

 replaced by such silica. The chert nodules are often several 

 feet in length, but generally they range in size from about an 

 inch to a foot in length. A very common form is oval or nearly 

 circular in random cross sections. The color of the chert is much 

 darker than the limestone in which it is imbedded. The rock 

 is otherwise remarkably free from impurities, weathers to a dull 

 gray color and forms but a scant regolith.* The outcrops are 

 usually abrupt and the ridge tops are bare. This formation 

 being at the top of the local geological column and its upper 

 limit being determined by erosion, its full thickness can not be 

 here ascertained. There remains of it, however, not less than 

 500 feet. 



A specimen of the chert from the Ruth limestone showing 

 a laminated structure was particularly studied. It consists of 

 blue-gray chert in layers from 3 to 6 mm. thick alternating with 



* Merrill, G. P. Koeks, Bock- Weathering and Soils, 1897, p. 299. 



