EROSION OF THE COLORADO PEATEAU. 45 
In the interval between Fort Defiance and the Rio Grande is another great volcanic moun- 
tain—Mt. Taylor (‘‘San Mateo’’)—which, like that of San Francisco, has burst through the 
sedimentary strata and poured over them floods of lava, which are as fresh as though ejected 
but yesterday. 
I have said that the Lower Cretaceous mesa was the highest of the table-lands which we 
passed over, and yet another must be added to the series before my description of them will 
be complete. 
On our route across the continent we passed somewhat south of the centre of what we may, 
perhaps, properly call the basin of the upper Colorado, and did not therefore mount quite to 
the summit of its geological series. Going north from the Moquis villages, on the Lower Creta- 
ceous Mesa, our progress was arrested by a want of water; the surface being everywhere cut 
by deep caiions, by which it is drained to excess; every rain drop which falls finding its way 
immediately into the bottom of these ravines, where it is hurried off to the far deeper caiions 
of the Colorado and its larger tributaries. Before we turned back, however, we had approached 
nearly to the base of a wall rising abruptly from the mesa in which we stood to the height of 
more than 1,000 feet. This wall was as white as chalk, and reflected the sunlight like a 
bank of snow. It is evidently the edge of another and higher plateau, and apparently reaches 
to the Great Colorado, where it caps the ‘‘high mesa,’’ forming part of the stupendous mural 
faces, presented toward the south and west, which were distinctly visible when we had receded 
from them to the distance of a hundred miles. 
What is the character of this upper mesa I had no means of determining at this time, and 
even now there may be some question about it; but I have scarcely a doubt that it is composed 
of the Upper Cretaceous strata, the equivalents of the ‘‘white chalk’’ of Europe. 
EROSION OF THE TABLE-LANDS. 
The sketch which has been given of the table-lands of the upper Colorado, though brief, 
will perhaps suffice to convey an idea of the generalities of their structure and relations. But 
before returning to the details of the local geology of our route, I ought perhaps to refer briefly 
to two questions of general import, which would naturally suggest themselves to any geologist 
who should traverse the table-lands west of the Rocky mountains, or should receive an accurate 
description of them from others. 
The first of these questions is: To what cause is due the peculiar topographical features of 
the surface of the table-lands—where the different formations succeed each other in a series of 
steps, which generally present abrupt and wall-like edges—the more recent strata oceupy- 
ing the highest portion of the plateau? The other has reference to the place and extent of 
the dry land, of which the erosion furnished the sediments now composing the table-lands. 
The first of these questions belongs appropriately to the subject of surface geology, and will 
be referred to again. I may say here, however, that, like the great cafions of the Colorado, the 
broad valleys bounded by high and perpendicular walls belong to a vast system of erosion, and 
are wholly due to the action of water. Probably nowhere in the world has the action of this 
agent produced results so surprising, both as regards their magnitude and their peculiar char- 
acter. It is not at all strange that a cause, which has given, to what was once an immense 
‘plain, underlaid by thousands of feet of sedimentary rocks, conformable throughout, a topo- 
graphical character more complicated than that of any mountain chain; which has made much 
of it absolutely impassable to man, or any animal but the winged bird, should be regarded as 
something out of the common course of nature. Hence the first and most plausible explana- 
tion of the striking surface features of this region will be to refer them to that embodiment of 
resistless power—the sword that cuts so many geological knots—volcanic force. The Great 
Caiion of the Colorado would be considered a vast fissure or rent in the earth’s crust, and the 
abrupt. termination of the steps of the table-lands as marking lines of displacement. This 
