46 GEOLOGY. 
theory though so plausible, and so entirely adequate to explain all the striking phenomena, 
lacks a single requisite to acceptance, and that is truth. 
Aside from the slight local disturbance of the sedimentary rocks about the San Francisco 
mountain, from the spurs of the Rocky mountains, near Fort Defiance, to those off the Cerbat 
and Aztec mountains on the west, the strata of the table-lands are as entirely unbroken as when 
first deposited. Having this question constantly in mind, and examining with all possible care 
the structure of the great caiions which we entered, 1 everywhere found evidence of the ex- 
clusive action of water in their formation. The opposite sides of the deepest chasm showed 
perfect correspondence of stratification, conforming to the general dip, and nowhere displace- 
ment ; and the bottom rock, so often dry and bare, was perhaps deeply eroded, but continuous 
from side to side, a portion of the yet undivided series lying below. 
The mesa walls should be included in the same category with those of the cafions; some 
times indeed they are but the sides of cafions miles in breadth. The origin of the series of 
escarpments which are met with in crossing the table-lands from west to east, is I think de- 
pendent upon very general but yet appreciable causes, to which I can here however but briefly 
allude. 
From the Cerbat mountains to the base of the high mesa the strata composing the table- 
lands have a northeasterly dip of about 100 feet to the mile. There they rise, but soon dip 
again into the valley of the Little Colorado, their strike being nearly at right angles with the 
course of the great draining stream, the Colorado. By a glance at the map it will be seen that 
the water shed made up of the San Francisco group the Mogollon, and spurs of the Rocky 
mountains, which throws the water into the Colorado over the table-lands from the south, 
southeast, and east, forms a semicircle imperfectly parallel with the course of the Colorado, 
into which the drainage from the different parts of this semicircle falls nearly at right angles. 
The flow of water from the mountains has therefore been here, as before, along the strike of the 
strata, north and northwest from the San Francisco and Mogollon mountains, on the western side 
of the basin. The legitimate and inevitable effect of this combination of causes has been to erode 
the softer down to the harder strata, forming broad valleys bounded on the west by the denuded 
slope of the harder rocks ; on the east, by the abrupt wall of the softer strata, most precipitous 
when capped by harder material. The erosion, for the most part produced by water flowing 
from a distant source, has taken place only at the bottom of each trough on the harder material, 
and thus has preserved the abruptness of the wall.—(See Section, p. 77.) 
This theory fully explains the erosion of the Carboniferous strata near Diamond and Cascade 
rivers, and the formation of the mesa wall of the Gypsiferous rocks bordering the valley of the 
Little Colorado. 
The Cretaceous mesas near the Moquis villages are eroded by the drainage from the spurs of 
the Rocky mountain system north of Fort Defiance; also across the strike of the strata, which 
here rise toward the southwest. 
The absence of these strata from all the interval between the Moquis villages, and the borders 
of the basin on the southeast, south, southwest, and west, indicate the immense erosion which 
the region hassuffered. It is, however, more than probable that as the basin was partially filled 
by the older rocks, the area over which the more recent strata were deposited became more and 
more restricted. 
West of Fort Defiance and east of the Moquis villages, in ‘‘Cafion Bonita,’’ Navajo valley, &c., 
we have examples of erosion, similar in all their general features to those already described, 
produced by streams running nearly north and south, and emptying into the Little Colorado. 
So near their sources these streams had less volume and power than farther westward, and have 
not so deeply scoured the plateau. 
I use the past participle in speaking of some of the streams whose erosive action has been so 
— from the remarkable fact that any of these eroded valleys are now dry; and in others 
