1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 351 



Four species of fresh-water molluscs have been recognized by the 

 writer in the collections from lone and Stewart valleys. It is doubtful, 

 however, whether our present knowledge of the geologic range of fresh- 

 water molluscan species occurring in the Tertiary rocks of the west 

 is as yet sufficiently complete to make it possible to use these forms for 

 definite age determinations of beds containing them. When identical 

 fresh-water species occur at two or more separated localities, however, 

 and these species occur in the same abundance relative to each other 

 at each locality, it appears highly probable that the containing beds 

 can be safely considered as equivalent. The writer has identified the 

 following forms in the collections from the Esmeralda beds in lone 

 and Stewart valleys: Helisoma eordillerana Hannibal; Viviparus 

 txirneri Hannibal ; a gastropod, probably a new species, but resembling 

 Melania scidptilis Meek, described from the Kawsoh Mountains west 

 of Carson Sink ; and a peleeypod resembling Corneocyclas meeki 

 Hannibal, described from Hawthorne, Nevada. The first two species 

 are the most abundant forms in the collections from Stewart and 

 lone valleys. They also occur most abundantly in collections taken 

 from the type section of the Esmeralda beds forty miles to the south- 

 ward. It is because of this identity of species, their relative abund- 

 ance, and the fact that the beds in lone Valley were traced through 

 continuously to the type Esmeralda locality, that the beds in lone 

 and Stewart valleys are identified with the Esmeralda formation. 

 These two species also occur most abundantly in collections obtained 

 from beds near Hawthorne, forty miles to the westward of Stewart 

 Valley, and it would seem that the Hawthorne beds can also be cor- 

 related safely with the Esmeralda on this palaeontologic evidence. 



The exact age of the Esmeralda formation has heretofore been a 

 matter of doubt. The definite determination of the age of the beds in 

 Stewart and lone valleys and the correlation of these beds with those 

 of the type Esmeralda, by similarity of molluscan fauna and by areal 

 continuity, fix the age of the Esmeralda as in or near upper Miocene. 



The writer was unsuccessful in an attempt to obtain mammalian 

 remains from the Siebert tuffs. 7 which lie to the east at Tonopah and 

 Goldfield. Parts of the Siebert section are identical in appearance 

 with strata in the Esmeralda, but their correlation with other Tertiary 

 sections in the basin has not as yet been established on palaeontologic 

 evidence. 



' Spurr, J. E., Geology of the Tonopah Mining District, Nevada, U. S. Geol. 

 Surv. Prof. Paper, no. 42, p. 51, 1905. 



