REVIEWS 371 
and young rhyolite, Pliocene. Each consists of a series of superimposed 
flows. 
The old rhyolite occupies a central area about 8 miles square, has 
a thickness of nearly 5,000 feet, and is the principal ore-bearing rock. 
Its flows are heavy, ill defined, and lie nearly horizontal. It is gently 
folded, faulted, and considerably fissured. It is coarse textured with 
large phenocrysts of wine-colored quartz and feldspar. 
The young rhyolite surrounds the old rhyolite with wide extent. 
It has a maximum thickness of nearly 2,000 feet. It is non-ore-bearing 
and but little disturbed. Its flows are relatively thin, well banded, and 
dip gently outward. It is lithoidal or aphanitic with little or no visible 
quartz, the silica occurring mainly in the form of tridymite. 
The principal rocks of the Contact district are the granodiorite 
and the Paleozoic sediments which it intrudes. The granodiorite 
occupies a central elongated quaquaversal east-west batholitic belt 
25 miles long by 6 miles wide. Surrounding it in a belt several miles 
in width succeeds the overlying quaquaversally outward dipping Paleo- 
zoic sediments with a known thickness of about 1,600 feet. They are 
considerably contact metamorphosed by the magmatic intrusion of the 
granodiorite and both they and the granodiorite are intruded by com- 
plementary syenitic, aplitic, lamprophyric, and monzonite dikes. The 
rhyolite which occurs chiefly peripherally corresponds to the young 
rhyolite of the Jarbidge district with which it seems to be continuous. 
The old rhyolite of that district seems to thin and peter out on the western 
headwaters of the Salmon and does not appear in the Contact district. 
The Tertiary lake beds of the area occur chiefly in low places in the 
Contact district where they have a known thickness of 400 feet. They 
are mainly gray “sandstone” composed of volcanic tuff which is chiefly 
pumiceous. In places they are tilted, flexed, and gently folded. They 
seem to be of Pliocene age and belong to the Humboldt formation. 
The Quaternary deposits of the area include in the Jarbidge Moun- 
tains, besides recent stream gravels, débris, and wash from the hills, 
also some glacial accumulations of Pleistocene age. 
Ore deposits—The ores were deposited in at least two distinct 
periods of mineralization, Cretaceous (?) and post-Miocene. 
The Cretaceous (?) deposits are chiefly auriferous and argentiferous 
copper ores. They occur mainly in the Contact district and consist 
principally of contact metamorphic deposits conforming to the contact 
zone of the granodiorite with the Paleozoic sedimentaries, and deposits 
in fissures. The contact metamorphic deposits contain much axinite, 
