172 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
STRUCTURE. 
The Amargosa Range lias been subjected to two main periods o 
folding and uplift, the earlier in pre-Tertiary and presumably post 
Jurassic time, the later probably in late Miocene time, after the depc 
sition of the Siebert lake beds. In the northern part of the range thi 
later folding was somewhat earlier than in the southern part and ap 
pears to have succeeded the outflow of the earlier rhyolite and pre 
(•((led the deposition of the Siebert lake beds. North of Wahguyh 
Peak the Paleozoic rocks and the earlier rhyolite are so folde< 
together that the differentiation of the two periods of folding is dif 
ficult. Even here, however, the Paleozoic rocks are more profoundly 
folded. The earlier folding was accompanied by reversed faulting 
and probably in the last stages by normal faulting; the later folding 
was accompanied by normal faulting. 
The major direction of the early folds was parallel to the trend o 
the range, but accompanying them are cross folds, which west o 
Grapevine Peak equal in importance and south of Indian Pass pre 
dominate over the northwest-southeast folds. In the vicinity o 
Grapevine Peak these cross folds were intensified by later folding. 
In the central portion of the range there is an anticlinorium witl 
northwest-southeast axis, composed of two main anticlines with \\ 
syncline between. The syncline at Keane Spring is comparatively 
narrow; at Boundary Canyon it is broader and more open; and ii 
the canyon in which Tule Spring is situated it is represented by twe 
minor synclines and an intervening anticline. At Chloride Cliff th< 
syncline is broken by a normal fault. Many of the minor folds an 
closely appressed isoclinals or fan folds, which in some instance: 
have horizontal axes. The passage from isoclinal folds to revers< 
faults is by no means unusual. South of Indian Pass the structun 
is that of an anticlinorium with northeast-southwest axis, of which th< 
long northwest limb dips rather gently to the southeast, while th< 
shorter southeast limb dips somewhat more steeply to the northwest 
Superimposed upon the main folds are minor folds, many of whicf 
are rather steep. The Pogonip limestone north of Grapevine Springs 
is folded into a gentle arch with east-west axis. 
Before the later folding was inaugurated the range was won 
down to an inconspicuous ridge, and the eastern slope and the whofi 
range south of the area mapped formed the bottom of the Miocene lake 
It is to the later folding and succeeding minor uplifts that the range 
owes its height. South of Daylight Spring the rhyolites dip at angle* 
of 20° to 40° away from the range, while the Siebert lake conglom- 
erates in this vicinity have a dip away from the range which in souk 
instances reaches TO . North of Daylight Spring the earlier rhyolite 
is folded into a synclinorium whose axis strikes N. 70° E., with itf 
trough situated about i miles north of Grapevine Peak. Normal 
