54 GEOLOGY. 
FIRST PLATEAU OF THE TABLE-LANDS. 
Emerging from the cajion of Yampai creek, we found ourselves on a plain extending indefi- 
nitely toward the north, and rising gently toward the west, where its broken edge forms the 
limit of Yampai valley. On the south it is bounded by mountains, of which the relations were 
not perceptible in the distance. On the east itis partially enclosed by trap hills, north of 
which our view was limited by the abruptly cut edge of another and higher plateau. 
Traversing this plain in a northeast direction we unconsciously passed the edge of the trap, 
and found ourselves on the somewhat irregular surface of beds of rounded gravel. The 
drainage of most of this surface is toward the north, and in that direction the arroyos become 
deeper and expose thick beds of sand, gravel, and boulders; the greater part of the pebbles 
being of a hard bluish-gray limestone. Ultimately the beds of the draining streams were 
found to have reached the rocky floor beneath. This they soon begin to erode, forming cajions 
which lead toward the Colorado. These caiions gradually increase in number and depth till 
twenty miles north of the point where the denudation commences, the plateau is intersected 
by a labyrinth of chasms from 2,000 to 3,000 feet deep, of which the nearly perpendicular 
walls forbid the passage of man or any other wingless animal. 
The plain which has just been described is little else than a broad valley of erosion, scooped 
out of the sedimentary rocks by the flow of water from the mountains south of it 
Its rocky basis is what I have regarded as probably the Lower Carboniferous or ‘‘Mountain 
limestone,’’ though no conclusive evidence of its age has yet been obtained. It extends to the 
border of Yampai valley, and, I have supposed, forms the greater part of the perpendicular 
section seen from below. 
CANON OF DIAMOND RIVER. 
Near Camp 66 we descended to the bottom of one of the arroyos I have mentioned, where 
it has a depth of more than 500 feet. A mile north of this point its sides converge and become 
perpendicular, forming a cafion of more magnificent proportions than any we had seen, and in 
geological interest far surpassing anything I had dared to hope for. 
This seemed to be our only avenue of approach to the Colorado, and we followed it for 
fifteen iles, to its junction with the still grander cafion of that stream. 
hou this distance the dry stream-bed on which we travelled descended at an average 
aay to the mile; while the summits of the cliffs on either side held nearly a 
_ All the strata here exposed have a strong dip to the northeast. In the 
velled we estimated it at fifty feet to the mile. 
In traversing AGS cafion, which joins that of Diamond river just before it reaches the Colo- 
rado, I was able to examine each stratum as we came down to the point where it was cut 
across}; and in that way make a very complete and satisfactory analysis of the series of rocks 
sing this first, and lowest, plateau of the table-lands. 
s be eat CANON OF " COLORADO. 
“ yrado at the mouth of Pianond river is but a portion of the stupen- 
=e its waters have cu in the strata of the table-lands, and of which a general 
lready been given. At this point its walls have an altitude of over 3,000 feet 
= t stream is. about 1 200 feet above the level of the 
eet h in the Black cafion. A few miles further east, where the surface 
ands ny an altitude of nearly 7,000 leek, <= ees of the cafion become far 
| and i its cliffs r rise to the height of more than a mile above the river.* 
Tors ® See section of cafion at this point, Chapter V, p. 42. 
