(,)i ati:knai;v DiorosiTs. ^\ 



pcriiMl. Ill iiiMiiy \va\s tlic\ \>vnv tlic closest rcsciiililaiicc in tlicir iihhIc dl' 

 occuiTcnrt', to similar la\as clscwlicic in tlir (Jrcat IJasin, where ex idciicc of 

 their agi; has been determined l)y their relation to sedimentary strata cariy- 

 inji" a Miocene or Pliocene fauna or flora. In mineral and chemical coinposi- 

 tion the lavas show jireat variations, liornlileiide-andesite, dacite, rliyolite, 

 pyroxene-andesite, and basalt l)eing well represented, with a wide range in 

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

 tiieir relations to each other, as well as their geological relations to the 

 orogra[)hic blocks, will lie found in the chapter devoteil to a discussion of 

 the Tertiar\- rocks. 



QrATEKN.\i;V DICI'DSITS. 



Quaternary Valleys.— The Eureku Mountains rise out of a broad plain 

 everywhere covered by Quaternary dei)osits that stretcli a\va\ in all direc- 

 tions far beyond the limits of the j)resent survey. The atlas siieets accom- 

 panying this work fail to indicate the relative area occupied 1)\ the moun- 

 tains to that <if the desert plains, l)ut an extension of the map onlv a few 

 miles more on all sides would at least liave shown how comp'eteK the 

 mountains were suitouiuIimI by a broad exi)anse of the so-called sau'e-lirush 

 deserts. With a single exception these l)road plains ojk'Ii one into the 

 other, the only !)arrier l»eing the diamond Mountains, wdiicli se|)arate Dia- 

 mond Vallev from X(;wark Valley. 



Newark \'alley and Fish (Jreek iJasin are simplv extiiiisions of the 

 same great j)lain, the former situated on the east and the latter on the .south 

 of the Eureka Mountahis. The Fish Creek Basin connects, ])v means 

 of a narrow [tass south of the Fish ("reek Mountains, with Antelope Valley, 

 a few miles beyond the limits of the map. .Viitelope \alle\- may be re- 

 garded as a southern extension of the broad, desert-like expaii.se of Hayes 

 Valley, whi(;h stretches far toward the north on the west side of the Pifion 

 Range. Hayes Valley connects with Diamond Valley by the narrow 

 gorge known as Tlu; Gate, which is simply a low jjass cut down to the 

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

 the latter. 



Little time has been devoted to the investigation of the Quaternary 

 get>logy in the immediate region of Eureka, but so far as the deposits have 



