348 



University of California Publications in Geology 



[Vol. 8 



assumption there is little evidence. It would be somewhat difficult 

 to estimate the original thickness, before reduction through erosion, 

 of the Esmeralda beds in Stewart and lone valleys. It is also not 

 clear what the thickness may even now be under the later deposits 

 in such areas as Gabbs Valley and Big Smoky Valley, in which depo- 

 sition may have gone on almost continuously since the beginning of 

 lacustral sedimentation in Miocene time. 



Stratigraphic Relations. — The relation of the Esmeralda strata to 

 the terranes they overlie is everywhere that of a notable unconformity. 

 Between these beds and the older limestones and intrusive rocks of 

 the surrounding ranges the unconformity is a profound one and is 

 plainly indicative of a long interval of geologic time. The hiatus 

 between the Esmeralda and the underlying lavas and agglomerates is 

 not such an important break. While the upper surface of the basic 

 lavas and agglomerates is seen to be an irregular erosion surface, as 

 shown on the western border of Stewart Valley, the dips in the lavas 

 are usually not very much steeper than those in the overlying lacus- 

 tral beds. The post-basic-lava erosion interval was certainly much 

 shorter than that represented by the break between the lavas and the 

 older limestones and intrusives. 



No evidence was obtained for considering that an important body 

 of Tertiary sediments of greater age than the Esmeralda underlies 

 these beds in the middle of Stewart or lone valleys. 



Where later lavas overlie the lacustral beds, the relation appears 

 to be an unconformable one. The fanglomerate mantle which caps 

 the lacustral beds in the mesa areas is everywhere uneonformably 

 upon them. 



Structure. — The Esmeralda strata have been moderately deformed 

 since their deposition, along structural axes roughly parallel to the 

 present ranges; i.e., roughly north and south. The folds are open and 

 not very persistent. One notes, perhaps, six or seven such low arches 

 in crossing Stewart Valley westward from the Nevada mine. The 

 dips seldom exceed ten to fifteen degrees. Because of the notable 

 rising and plunging of the deformational axes, the figures traced by 

 outcropping beds are markedly irregular. Where they rest against 

 the older rocks of the mountain flanks or against outlying masses of 

 the older rocks in the valleys, the strata often dip steeply, and locally 

 are eve]1 overturned. They almost always dip away from the older 

 rocks at such contacts, regardless of the general direction of dip of 

 the beds in the neighborhood. 



