TRIASSIC. 521 



(over 1,000 feet) of andesitic and rhjrolitic lavas, with 1,500 feet of overlying shales, sand- 

 stones, and tuffs of Triassic age, comes between the horizon of the Robinson formation and 

 that of the Hosselkus limestone. It is evident, therefore, that in the Taylorsville region there 

 is a decided interruption between the Hosselkus limestone and the Robinson formation. It 

 is possible, however, that their contact in the Taylorsville region is a plane of displacement, 

 and that the formations of the Redding region missing in the Taylorsville region may, in part 

 at least, be thus accounted for. 



The Swearinger formation is composed chiefly of dark slaty shale, sometimes becoming 

 more or less calcareous and at others decidedly siliceous, but the thin beds of limestone or 

 chert form only a small proportion of the whole mass. In the side of the Swearinger slate 

 adjoining the Hosselkus limestone thin lenticular beds of limestone become more abundant. 

 They are generally dark, with irregular cherty or sandy layers, and fossihferous. 



The greatest thickness of the Swearinger slate exposed along Genesee Valley, where meas- 

 ured, is about 200 feet, but the amount cut off by the granodiorite or covered by the over- 

 lapping Trail formation we have no means of estimating except by comparison with forma- 

 tions of the same horizon in other regions; such comparison indicates that the thickness is 

 probably not over 400 feet. 



The fossils of the Swearinger slate were discovered and described by the Geological Survey 

 of CaHfornia under Prof. J. D. Whitney. Our knowledge of the fauna was greatly extended 

 by Prof. Alpheus Hyatt, who has pubUshed'^ lists of the forms found in the subordinate pale- 

 ontological horizons within the formation. They need not be repeated here except to note 

 that the form from these beds once regarded as Monotis suhcircularis is now considered by 

 J. P. Smith and others to be Pseudomonotis. Prof. Hyatt regarded the Swearinger slate as 

 belonging to the upper Triassic, equivalent to the upper Noric of the Alpin.e Triassic, and as 

 far as I am aware this reference has not been changed essentially by the somewhat later 

 researches of Prof. Smith. 



J-K 10-11. NEVADA AND EASTEBN CALIFORNIA. 



The Triassic section of Nevada was originally described by King*"'* in the 

 following summary of the data stated more at length on other pages of his report. 



Passing now to the district of western Nevada, the sections, which often do not reach 

 the base of the conformable series, expose two distinct, easily recognizable groups of the Trias. 

 The Koipato, already described, is made up of siliceous and argillaceous beds, whose chemical 

 peculiarity is the almost total absence of soda and lime and the high percentage of alumina 

 and potash — a series probably derived from the disintegration of the heavy Weber Carbonif- 

 erous quartzite, which must for a long time have constituted the main surface of the erosion 

 of the newly lifted Mesozoic land. This series has an observable thickness of about 6,000 feet, 

 with an unknown quantity to be added for the bottom unseen beds. Conformably over the 

 Koipato is the great Alpine Trias Star Peak series of 10,000 feet, composed of an alternation 

 of three great limestone zones and three interposed quartzite zones, the lower quartzite closely 

 following the physical and chemical pecuUarities of the Koipato series below, the upper two 

 quartzites representing moderately pure siliceous sediment. The fossils of these limestones, 

 as already described, repeat with marvelous exactness the facies of the St. Cassian and Hall- 

 stadt beds of the Austrian Alps. 



Smith ^^** states that beneath the strata of Middle Triassic age in Inyo County, 

 southeastern California, occur Lower Triassic beds. He says: 



In 1896 Dr. C. D. Walcott discovered some ammonite-bearing limestone in Inyo County, 

 CaL, on the east side of Owens Valley, 10 miles east-northeast of Lone Pine, 3 miles southeast of 



a Bull. Geol. Soc. America, vol. 3, 1892, pp. 397-400. 



