24 GEOLOGY OF THE EUEEKA DISTRICT. 



River to the Eureka ]\Iouiitains, may be said to teriiiiuate at the deeply 

 ei'oded pass kuowu as The Gate, as it there loses its distinctive features. 

 The mouoclinal character of the uplifted ridges is, however, still maintained 

 nearly to Spanish Mountain, or until cut off by the Spring Valley fault. 



From Dry Lake westward the mountains rise abruptly, frequently in 

 steep cliffs, presenting a somewhat monotonous aspect of dark bluish gray 

 limestone covered with a scanty groAvth of mountain mahogany (Cercocarpus 

 Icedifolius), from which the region derives its name. A few culminating 

 points attain elevations above the general level, but these gradually fall 

 away to the westward in long uniform ridges, sharply defined by drainage 

 channels that cut down hundreds of feet into the limestones with nearly 

 vertical escarpments. 



Mahogany Hills are made up for the most part of Nevada limestone, 

 which everywhere forms all the more elevated portions. Silurian rocks 

 occur in one or two localities, but principally at Spanish Mountain, where 

 the Eureka quartzite is admirably shown, with all its peculiarities of struc- 

 ture, overlain by the Lone Mountain limestone, which in turn j^asses con- 

 formably into the Nevada limestone. For purposes of stratigraphical geol- 

 ogy, the position of Spanish Mountain is most fortunate, as its relation to 

 the overlying Devonian limestone is well brought out, while its relation to 

 the underlying limestones and shales of the LoAver Silurian and Cambrian 

 is demonstrated beyond question in both the Fish Creek Mountains and 

 Prospect Ridge. Spanish Mountain happens to be the only area of Eureka 

 quartzite in the Mahogany Hills. On the southern slope of Comb's Peak 

 the upturned beds afford an excellent exposure of the limestones overlying 

 the Eureka quartzite, and give a section of Lone Mountain rocks lower than 

 found elsewhere, including a series of beds whose geological position is 

 determined by a characteristic Trenton fauna. The relationship of this fauna 

 just above the Eureka quartzite to the fauna found elsewhere immediately 

 below the quartzite offers an important link in the paleontological history 

 of the Eureka District. One of the best sections across the Nevada limestone 

 may be found on the ridge north of Modoc Peak, Avhere the beds tlu'oughout 

 a great vertical thickness present a nearly uniform strike and dip, with but 

 little disturbance or dislocation. The Modoc section measures about 5,400feet 



