CARBONIFEROUS. 63 



The paleontological information, therefore, is so far a confirmation of 

 the suggestion offered above from lithological composition, viz, that the 

 Cambrian beds are here not more than fifty to a hundred feet thick (a 

 notable decrease from the estimated 12,000 feet in the Wasatch, or from the 

 more definitely-determined thickness given by Mr. A. Hague for Eureka, 

 Nevada, of 7,700 feet), and that the limestone beds above are Silurian. 



CARBONIFEROUS. 



The beds of this period are, as in other parts of the Rocky Mountain 

 region, more fully developed and more abundant in fossil remains than 

 those of the other Paleozoic horizons. The Carboniferous period here, as 

 in the Wasatch, consisted of two limestone-making epochs, separated by a 

 long period of silicious deposits, with the difference that in the shallow seas, 

 in which the Carboniferous of the Mosquito Range was formed, detrital and 

 silicious deposits predominated over calcai'eous deposits. The series, there- 

 fore, lends itself to a triple subdivision into lower, middle, and upper Car- 

 boniferous, which are here assigned to it mainly on lithological grounds, since 

 our knowledge of the Carboniferous fauna of the Rocky Mountain region 

 is not yet sufficiently complete to enable us to establish satisfactory paleon- 

 tological subdivisions, and many forms considered characteristic of the Coal 

 Measures of the East range from the bottom to the very top of the series. 



Blue or ore-bearing Limestone. — The bcds included Under tliis local name, 

 which are designated on the map by a deep-blue color (e), and which, from 

 the fact that they form the ore-bearing rocks par excellence of the region, it 

 is most important to be able to trace accurately, are fortunately marked by 

 persistent and characteristic features. They have an average thickness of 

 about 200 feet. In color they are of a deep grayish-blue, often' nearly 

 black in the upper portion of the series, while some of the lower beds are 

 lighter in color, approaching a drab, and, where locally bleached, difficult 

 to distinguish lithologically from the underlying White Limestone. The 

 upper bed is well marked by characteristic concretions of black chert, fre- 

 quently hollow in the center and often containing within their mass dis- 

 tinct casts of fossils. Owing to their superior resistance to atmospheric 

 agencies, they are often weathered out and left in nodular masses of irreg- 



