200 GEOLOGY OF THE EUREKA DISTEICT. 



heaviest not over 100 feet in thickness. It is not singular that in a massive 

 bed of limestone it becomes a difficult matter to divide the Silurian, 

 Devonian, and Carboniferous with any degree of precision. Between the 

 Silurian and Devonian at the base of the section it is impossible to draw 

 any line of demarcation. It is safe, provisionally, to place 1,000 feet of the 

 light gray limestone in the Lone Mountain period, leaving strata carry- 

 ing Stromatopora and branching corals included in the Devonian. In the 

 390 feet of bluish black limestones (No. 5 of the section) a marked 

 Devonian fauna occurs at the base and Lower Carboniferous fauna at the 

 summit, without any change in the lithological character of the beds. Pro- 

 visionally the line is drawn so as to include in the Carboniferous all beds 

 carrying Spirifera lineata and Spirifera cristata, and leaving Atrypa reticularis 

 in the Devonian. By this division we have the following thicknesses for 

 the different periods: 



Feet. 



Carboniferous 2, 160 



Devonian 5, 400 



Silurian 1, 000 



Pinon Range. To these sections south and southeast of Eureka may be 

 added still another, constructed across Pinon Range about 60 miles north- 

 ward. This range is a long, narrow ridge, stretching from the Humboldt 

 River southward until it joins the Eureka Mountains at The Gate, the 

 southern end of the range coming within the area of this survey. 



The Pinon Range attains its greatest elevation just south of the Hum- 

 boldt, where the best continuous sections of the Lower Paleozoic rocks 

 occur. The range was crossed at several points by the geologists of the 

 Fortieth Parallel Exploration, the Devonian rocks being traced by the writer 

 for nearly their entire length from The Gate to the Humboldt River. At 

 the request of the writer, Mr. Walcott visited the northern end for the pur- 

 pose of a comparative study of the section exposed at Ravens Nest with 

 the corresponding rocks at Eureka. Here the beds strike obliquely across 

 the trend of the range from the northwest base of Ravens Nest to the base 

 of Pinto Peak, the course of the range being approximately north and 

 south. 



