MAHOGANY HILLS. 23 



quartzite overlies the limestone on both sides of the mountains, but as the 

 dip of the strata coincides closely with the inclination of the western slope, 

 it comes to the surface only near the base of the ridge. As the strata dip 

 away both to the north and south from the central body of Pogonip lime- 

 stone, a belt of the quartzite may be observed encircling it on all sides. 

 Nowhere do the Fish Creek Mountains expose a section of the Pogonip 

 limestone for more than one-quarter of its thickness, as given in the general 

 section, although numerous excellent partial sections are shown of the 

 Upper Pogonip beds. Northward of Bellevue Peak, and in the region 

 of Castle Mountain, the Lone Mountain, limestone overlying the Eureka 

 quartzite comes to the surface, and again at the southern end of the range, 

 but beyond the limits of the map. 



From this description, and by the aid of the map (atlas Sheet xi), a clear 

 idea may be obtained of the broader features of the Fish Creek Mountains, 

 and in the chapters devoted to the Silurian rocks and the descriptive geology 

 there will be found the evidences in detail for the conclusions presented 

 here as to their age and structure." 



Mahogany Hills. The Mahogany Hills are situated on the west side of the 

 Eureka Mountains. They occupy by far the largest area of any of the 

 mountain blocks into which the country has been divided, and are as sharply 

 denned as any of the others by natural physical outlines. Spring Valley and 

 Canyon serve as an excellent boundary between them and Prospect Ridge, 

 but everywhere else, except along the narrow belt which connects them with 

 the Fish Creek Mountains, the broad Quaternary plain rests against the 

 upturned edges of the outlying ridges. From Spring Valley the Mahogany 

 Hills extend westward, a mountain mass over 8 miles in width ; in a north 

 and south direction they present an unbroken body of limestone, 12 miles 

 in length. This broad mountain mass maybe divided into two nearly equal 

 parts, separated by the level plain of Dry Lake and the narrow gorge of 

 Yahoo Canyon, the lake at one time draining northward through the canyon 

 into Hayes Valley. The country to the east of the lake and canyon, 

 while it has much in common with the western side, is, in structural 

 features, closely related to the Pinon Range. This latter range, which is 

 made up of a number of longitudinal ridges extending from the Humboldt 



