MAHOGANY HILLS. 23 



(lUMi'tzite overlies the linicsronc on both sides of the inouiitiiins, Itut as the 

 di]) of the strata coineides (dosely witli 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 l)elt of the quartzite may be observed encircling- it on all sides. 

 Nowhere do the Fish Creek Mountams expose a section of the I'ogonip 

 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 Eiu'eka 

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

 but beyond the limits of the map. 



From this descrijition, 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 chaj)ters 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 Mahogauy Hills are situated on the west side of the 

 Eureka ]\Iountains. They occupy by far the largest area of any of the 

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

 defined as any of the others by natural physical outlines. Spiing ^^alley and 

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

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

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

 upturned edges of the (uitlying ridges. From Spring Valley the ]\Iahogany 

 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 di^■ided into two nearly equal 

 parts, separated by the level plain of Dry Lake and the narrow goi'ge 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 Pifion Range. This latter range, which is 

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



