ERODED VALLEYS, NAVAJO COUNTRY. 89 
churches, castles, gates, and monuments of various kinds. These are surrounded by a green 
and level plain, with which their various outlines and colors strongly contrast, the whole picture 
vividly recalling the ruined structures of the Old World. 
Like most of the other phenomena of erosion in the Colorado eountry, this great valley was 
principally formed in an epoch in which the annual amount of rain was much greater than at 
present, for no stream now flows through it. 
The strata which compose the cliffs which bound this valley include the lower portion of the 
Cretaceous formation with a greater thickness of the Marl series. Atthe base of the Cretaceous 
rocks is a group of brown foliated sandstones and beds of lignite, entirely undistinguishable 
from the upper part of the section exposed north of Oraybe and given on page 84. _ It is possible, 
however, that these beds of lignite form the equivalent of that containing the fossil plants at 
Camp 92; but I was not able to find any fossils to establish that parallelism. These lignite 
oatge ee Sa 
Fig. 24.—na’ BRIDGE, CAMP 100. 
beds are either in the base of the Cretaceous series or immediately beneath it; holding, there- 
fore, the place in the scale of that at Camp 92. If an identity could be established between 
these deposits we should here have something more than twice as many feet in thickness of 
what may be considered Jurassic rocks, as at that locality. A few miles further east this group 
entirely disappears, and nothing is there interposed between the base of the Cretaceous and 
the summit of the Marl series. aa 
In a great number of localities about Camp 100 the lignites have been burned out, giving a 
decided red color to the strata which enclosed them. This phenomenon is probably due to 
spontaneous combustion, being caused by the oxidation of the bisulphuret of iron, which is 
abundantly contained in all these carbonaceous strata. The same thing has taken place in the 
Cretaceous lignites east of Fort Defiance, and on a still grander scale in the same formation on 
the Upper Missouri. 
12——L 
