128 GEOLOGY OF THE EUREKA DISTRICT. 



only after long study of them that they can be readily distinguished; 

 along the line of contact it is by no means easy to separate them. Evi- 

 dences of geological position come in, however, and the limestone, both 

 above and below the Eureka horizon, usually determines the age of 

 the beds. As the country is much broken up by profound faults, and 

 the Eureka quartzite is not over 500 feet in thickness, either the Pogonip 

 below or Lone Mountain horizon above, frequently both, are apt to come 

 to the surface near the exposures of the Silurian quartzite. Wherever the 

 Cambrian quartzite is found it is overlain by Cambrian limestone. 



On the summit of the ridge along the line of the Prospect Peak fault 

 occurs a small patch of highly altered limestone, without any structural 

 indications of its relationship to either of the quartzite bodies. Its position is 

 difficult to explain satisfactorily, but it has been referred to the Pogonip, 

 since it more closely resembles the limestone of White Mountain than that 

 of Prospect Ridge. 



From Prospect Peak southward the Eureka quartzite forms the west 

 side of Prospect Ridge, following the line of the Sierra fault. The ridge 

 falls away steadily to the south for 1^ miles, with a descent of over 

 1,500 feet to Sierra Valley. A series of minor longitudinal faults pre- 

 sents a much more abrupt slope on the west side and prevents the 

 underlying formations from coming to the surface, notwithstanding that a 

 narrow ravine is eroded in the quartzites for nearly 700 feet in depth. Not 

 till descending the slope for nearly 1,000 feet do the Pogonip beds come 

 to the surface, and then only a small patch of this underlying rock is 

 exposed. This interesting body of limestone crops out to the northeast of 

 Lookout Mountain, where it presents an obscure exposure of slight area 

 and thickness. The fauna obtained here is strikingly Pogonip in aspect, 

 and resembles the fauna found on the face of White Mountain for 500 to 

 1,000 feet below the summit. Associated with other more common forms 

 are Raphistoma nasoni, Maclurea annulata, and Leperditia livia, all recog- 

 nized as belonging to the Pogonip of White Pine. The interest in this 

 identification lies in the fact that only a few hundred feet to the southward 

 the Cambrian limestone comes to the surface in Sierra Valley, while just to 



