GEOLOGIC HISTORY. 
level. The area under consideration was, indeed, so near the ancient 
shore line, naturally a zone of greatest differential uplift, that islands 
may have stood at times above the surface of the sen. 
Hague has found in the Eureka district a marked unconform- 
ity between the Eureka quartzite and the Lone Mountain lime- 
stone, and according to his description younger formations overlap 
the Eureka quartzite. Such an unconformity was not definitely 
recognized in the area surveyed, but it probably exists. The appar- 
ent absence of Devonian rocks has been already mentioned, and the 
alternative hypotheses are presented that the area was a land mas- 
during the greater portion of Devonian time, or that Devonian rocks 
were deposited and later eroded. Spurr found Devonian rocks Is 
miles south-southeast of Ash Meadows h and 5 miles east of Point 
of Rocks, r and his map shows large Devonian areas from 30 to 40 
miles "east of the area here considered. Whichever of these hypothe- 
ses may be true, it is evident that at some time between the deposition 
of the Lone Mountain limestone and the Weber conglomerate a land 
mass existed over a portion at least of southwestern Nevada and 
eastern California. Not only were the Cambrian, Ordovician, and 
Silurian rocks uplifted and eroded, but the limestones were cut by 
calcite veins and in part silicified to jasperoid, and the sandstones 
were cemented to quartzites prior to their inclusion as pebbles in the 
Weber conglomerate. Cementation is evident not alone from the 
presence of these pebbles, but also from the fact that the early Paleo- 
zoic quartzites are thoroughly indurated, while the Pennsylvanian 
sandstones are of more open texture. The earliest Carboniferous 
rocks exposed in the area studied are of Pennsylvanian age, and it is 
evident that during that period the area again formed the bottom of a 
sea, which, however, during the deposition of the Weber conglomer- 
ate could not have been far from a land mass, while a conglomerate 
in the Pennsylvanian limestone also indicates shallow-water con- 
ditions. After the deposition of this limestone the area was prob- 
ably lifted above the sea and never sank beneath it again. The 
period of Paleozoic sedimentation thus concluded was unattended 
by volcanism, with the possible exception of that which produced 
the metamorphosed basic igneous rocks of the Amargosa Range. 
King' 7 believed that contemporaneously with the emergence of 
this portion of the Great Basin above the sea the old land mass to 
the west subsided and that upon it were deposited Triassic and 
urassic rocks. He further believed c that the ranges of western 
evada were formed at the same time as the Sierra Nevada, or at 
« Hague, Arnold, Mod. U. S. Geol. Survey, vol. 20. 1892. p. 57. 
"Spurr, J. E., Bull. U. S. Geol. Survey No. 208, L903, pp. 197-198. 
<■ Op. Git, p. 101. 
d King, Clarence, U. S. Geol. Explor. 40th Tar., vol. 1. 1878, p. 7..!». 
e Op. cit, p. 747. 
