SILVER PEAK RANGE, NORTHEASTERN EXTENSION. 
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laid down in comparatively shallow water in which conditions favor- 
ing mechanical sedimentation on the one hand and organic and per- 
haps chemical sedimentation on the other hand alternated. 
Spnrr" found fossils southwest of Lida which were determined 
by Mr. C. D. Walcott to be of Lower Cambrian age. The limestone 4 
miles west-northwest of Montezuma Peak contains closely concen- 
trically laminated balls averaging one-half inch in diameter. Mr. E. 
O. Ulrich states that these resemble Girvanella, a calcareous alga, and 
are probably of Cambrian age. On the evidence of these fossils and 
the resemblance of the series to that at Cuprite the rocks are believed 
to be, in the main at least, of Lower Cambrian age. Turner h maps 
Ordovician rocks at one place near the border of the Silver Peak 
quadrangle, but such rocks, if they exist, were not differentiated 
during the present reconnaissance. 
Siebert lake beds. — Prior to the deposition of the Siebert lake beds 
the Cambrian rocks had a rugged topography. These beds, 17 miles 
north of Lida, are coarsely conglomeratic and inliers of limestone pro- 
trude through them and outliers of lake beds occur on the Cambrian 
bocks. The lake beds cover a considerable area on the western border 
of the region under consideration, a somewhat smaller area near Mon- 
tezuma Peak, and a small area 5 miles south of Alkali Spring. The 
northern boundary of the largest area is approximately correct as 
mapped. The southern boundary is less accurately drawn and some 
Tertiary igneous rocks may lie between the Cambrian rocks and the 
lake beds. 
These beds, which are composed largely of rhyolitic material, con- 
sist of typically incoherent, unevenly granular, well-bedded sand- 
stones. When unstained by iron, they are white in color, but brilliant 
:eds and yellows are characteristic of many areas. Fine-grained beds 
alternate with conglomeratic layers containing pebbles of Cambrian 
pocks, quartz-monzonite porphyry, diorite porphyry, rhyolite, and 
gneous rocks of andesitic and basaltic affinities. The pebbles are well 
rounded or semiangular. Microscopic examination of several slides 
|hows the presence of quartz and orthoclase crystals in a pyroclastic 
natrix. 
j Locally the Siebert lake beds have been indurated by silicification 
llong joints and in irregular bodies. Quartz, chalcedony, and opal 
|ll cavities. The silicified portions weather into wall-like masses 
ftlong joints and into grotesque topographic forms where silicification 
las been less regular. 
Rhyolitic and dacitic flows are mterbedded throughout the sedi- 
ments, although they increase toward the bottom of the formation 
nd dacite appears to underlie it. 
Turner. H. W., Silver Peak folio ; unpublished. 
"Spurr, J. E., Bull. U. S. Geol. Survey No. 208, 1903, p. 186. 
