XVI OUTLINE OF THIS' VOLUME. 



is easily distiuguished from both the coarso sands and grits of the Cambrian below and the Carbon- 

 iferous conglomerate above. An unconformity of deposition exists between the Eureka and Lone 

 Mountain epochs. Both the Trenton and Niagara formations are included within the Loue Moun- 

 tain epoch. 



Chaptek IV. By impercejitible gradations limestones of the Loue Mountain epoch pass 

 upward into those of the Devonian period. Devonian rocks occupy a larger area in the District than 

 those of any other period, and present a greater thickness than either the Cambrian or Silurian. 

 They measure 8,000 feet, divided into two epochs : A bluish limestone — the Nevada limestone — and an 

 argillaceous black shale — the White Pine shale. The limestone carries a rich invertebrate fauna from 

 base to summit. The black shale is characterized by a flora which, though fragmentary, is suffi- 

 ciently well preserved to identify the genera as belonging to the Upper Devonian. 



The Carboniferous rocks measure 9,300 feet, which, however, does not quite represent their full 

 development, the uppermost bods having undergone more or less erosion. They have been divided 

 into four epochs, as follows : Diamond Peak quartzite, Lower Coal-measure limestone, Weber con- 

 glomerate, and Upper Coal-measure limestone. As the limestone is in general favorable to the preser- 

 vation of organic remains, fossil-bearing strata occur throughout the beds. Three salient features 

 mark the life of the Lower Coal-measures. First, the occurrence near the base of the limestone of a 

 fresh-water fauna; second, the varied development of the Lamellibranchiates a class which has here- 

 tofore been but sparingly represented in the collection of Carboniferous fossils from the Cordillera ; 

 third, the mingling near the base of the horizon of Devonian, Lower Carboniferous, and Coal-measure 

 species in gray limestone directly overlying beds characterized by a purely Coal-measure fauna. 



In the first range to the east of the Eureka Mountains Carboniferous rocks extend for miles 

 along the edge of the valley, in which well developed coal seams occur. 



Chapter V. This chapter is devoted to the descriptive geology of the sedimentary rocks. 

 Each orographic block is described in detail, beginning with Prospect Ridge, where the oldest rocks 

 occur, followed by the othcir blocks according to the succession of strata. It gives a connected 

 description of the country and points out the relations of the different mountain masses to each other. 



Chapter VI. A discussion of the Paleozoic rocks follows, based upon the facts presented in 

 the earlier chapters. It is shown that during Paleozoic time a pre-Cambrian continent existed in 

 western Nevada which furnished to an ocean lying to the eastward an enormous amount of detrital 

 material. It is pointed out that the Eureka region was situated not far from the eastern border of this 

 land mass, and that a large part of its coarse conglomerates and mechanical sediments must have been 

 offshore deposits. The geological record affords proof of elevation and depression throughout Paleozoic 

 time with intervals of shallow water and proximity of land areas between periods of relatively deep 

 seas. Fresh-water life, plant remains, and coal seams at different horizons furnish additional evidence 

 of shallow water and offshore deposits. A study of Paleozoic rocks in other parts of southern and 

 western Nevada exhibit nearly similar geological conditions as regards sequence of beds. This is 

 especially well shown both at White Pine and in the Highland and Piiiou ranges. The sequence of 

 strata, both to the north and south, indicates a closer agreement with the conditions of sedimentation 

 at Eureka than the many exposiu-es situated but a short distance eastward of the latter area. The 

 structural relations of the different orographic blocks to each other and the outbursts of igneous rocks 

 are well brought out in cross-section. An instructive feature at Eureka is the close relationship 

 between the anticlinal and synclinal folds to the profound north and south faults. 



Chapter VII. Pre-Tertiary igneous rocks play a very subordinate part. They may be ilassed 

 under three heads: Granite, granite-porphyry, and quartz-porphyry. The granite occupies a limited 

 area on Prospect Kidge. Both the granite and quartz-por])hyries occur as dikes. Structural varia- 

 tions iu the dikes arc mainly dependent upon the chilling efj'ect of cold contact walls upon a rapidly 



