POST- JURASSIC EROSION INTERVAL 337 



How long this period of elevation and erosion continued has not been 

 definitely determined, because the time of the next succeeding event has 

 not been satisfactorily established. That the processes of elevation dis- 

 continued very long before the erosion period closed is evident from the 

 fact that a topography of very low relief was produced. In the Sierra 

 Nevada, it may be noted, the post- Jurassic upheaval was practically 

 brought to a close before the beginning of the deposition of the Chico 

 beds* (Upper Cretaceous). 



The first disturbance during this erosion period, of which a record has 

 been found, is that warping of the topography which produced the fresh- 

 water lakes. The oldest lake deposits thus far recognized in this dis- 

 trict are the Truckee beds. These were considered by the geologists of 

 the Fortieth Parallel Survey to be of Miocene age, but the evidence is 

 of an unsatisfactory character — chiefly some freshwater mollusks of 

 doubtful horizon, f and general deductions from the amount of deforma- 

 mation. The type locality, Kawsoh mountain, lies just at the south end 

 of the Humboldt valley. Observations made near Reno, Nevada, how- 

 ever, on what are presumably Truckee beds, indicate that these deposits 

 are probably Pliocene. 



We ma}' say, then, that the period of erosion extended without any 

 other notable contemporaneous event through the Cretaceous, the Eocene, 

 and part of the later Tertiary time. Then occurred the warping which 

 produced the lake or lakes in which the Truckee beds were deposited. 

 The country remained of rather low relief, and during some stages diat- 

 omaceous ooze was abundantly deposited over large areas. 



VOLCANIC ACTIVITY 



It was during this period of crustal warping, perhaps near its close, 

 that volcanic activity broke out in the form of explosive eruptions. 

 Judging from the relationships shown in the Trinity foothills, deforma- 

 tion took place after this tuff-forming period, and was followed by out- 

 pourings of rhyolite lava. Then, again, the country was covered with 

 the products of a series of explosive eruptions. % 



♦Turner : U. S. Geol. Survey, Seventeenth Annual Report, p. 547. 



t Report of the Fortieth Parallel Survey, vol. ii, p. 767. The fossils there given are Camifex 

 (vortifex) binneyi, C. (v.) Troyoni, Aneylus undulatus, Melania sculptilis, M. subsculptilis, Sphcerum? 

 rugosum, and S. idahoense. The report says, "They are all of decidedly freshwater types, and 

 have been referred by Professor Meek to the Miocene or later formations, but on structural 

 grounds, . . . they are regarded with but little hesitation as belonging to the Miocene age." 

 A Rhinoceros tooth is also reported, but of undetermined species. 



% The relationship of the Truckee beds and the rhyolites and turfs is not shown at any of the 

 localities studied during this investigation, but according to the Fortieth Parallel Survey, vol. i, 

 p. 644, the rhyolite distinctly overlies the lake beds, and Mr J. T Reid, of Lovelock, reports the 

 same sequence from the Trinity mountains. It is possible that the (sometimes tilted) tuffs under 

 the rhyolites were deposited conformably on the Truckee beds, and may be considered part of 

 that formation. 



XLIV— Bull. Geol. Soc. Am., Vol. 15, 1903 



