QI ATi;i!NAl;V I >K POSITS. 31 



period. In many ways they bear the closest resemblance in their mode of 

 occurrence, to similar lavns elsewhere in the Great Basin, when; evidence of 

 their age has been determined by their relation to sedimentary strata carry- 

 ing a Miocene or Pliocene fauna or flora. In mineral and chemical composi- 

 tion the lavas show great variations, hornblende-andesite, dacite, rhyolite, 

 pyroxene-andesite, and basalt being well represented, with a wide range in 

 structural and physical features. A description of these different lavas and 

 their relations to each other, as well as their geologic-i 1 relations to the 

 orographic blocks, will be found in the chapter devoted to a discussion of 

 the Tertiary rocks. 



QUATERNARY DEPOSITS. 



Quaternary Valleys. The Eureka Mountains rise out of a broad plain 

 everywhere covered by Quaternary deposits that stretch away in all direc- 

 tions far beyond the limits of the present survey. The atlas sheets accom- 

 panying this work fail to indicate the relative area occupied by the moun- 

 tains to that of the desert plains, but an extension of the map only a few 

 miles more on all sides would at least have shown how completely the 

 mountains were surrounded by a broad expanse of the so-called sage-brush 

 deserts. With a single exception these broad plains open one into the 

 other, the only barrier being the Diamond Mountains, which separate Dia- 

 mond Valley from Newark Valley. 



Newark Valley and Fish Creek Basin are simply extensions of the 

 same great plain, the former situated on the east and the latter on the south 

 of the Eureka Mountains. The Fish Creek Basin connects, by means 

 of a narrow pass south of the Fish Creek Mountains, with Antelope Valley, 

 a few miles beyond the limits of the map. Antelope Valley may be re- 

 garded as a southern extension of the broad, desert-like expanse of Hayes 

 Valley, which stretches far toward the north on the west side of the Pinon 

 Range. Hayes Valley connects with Diamond Valley by the narrow 

 gorge known as The Gate, which is simply a low pass cut down to the 

 level of the plain through which the former valley at one time drained into 

 the latter. 



Little time has been devoted to the investigation of the Quaternary 

 geology in the immediate region of Eureka, but so far as the deposits have 



