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104 | GEOLOGY. 
This will account for the profound depth of its excavation, or, in other words, for the great 
relative height of the sides of its trough. Why those sides are so often perpendicular, presenting 
mural faces a mile in height, and why they are not the gentle slopes of broad and fertile valleys, 
has been explained by reference to the facts that very little moisture in the form of rain falls 
on that surface; almost no vegetation is there to aid in softening the savage scenery, but the 
flow of water from the distant mountains, constant and copious, ever spends its energy in 
wearing away the rock beneath it in long and narrow lines. The walls on either side of the 
deepening beds of the streams, when beyond their influence, suffer little change, but stand as 
monuments on which are written the history of countless ages. 
The picture presented by the ‘‘Plains’’ east of the mountains where we crossed them is 
quite a contrast to this. The fall from the bases of the Rocky mountains to the sea level is 
spread over twice the distance, and distributed along a slope so gentle that its descent is 
invisible to the eye, and it seems in all directions level. All that slope is at least moderately 
well watered, and near the Mississippi, where the erosive action would have left its most con- 
spicuous marks, copious rains have washed them all away or stripped them of all character. 
It should be borne in mind that this description is only applied and is only applicable to 
that part of the prairies bordering the Santa Fé road. Both north and south of that line, though 
at considerable distances from it, we might find regions more like, in their physical aspects, 
geological structure and climate, to the western portion of the table lands. 
North of the Platte is a wide area much more dry and sterile than that adjacent to our route. 
The surface is not as high nor as level as the plateau traversed by the Colorado, and the fall of 
the stream is less rapid. Great cafions are consequently not found there. Carions of smaller 
size are not uncommon, and the banks of the stream generally present more abrupt faces than 
those on our route. 
South of the Santa Fé road the Llano Estacado (judging from the descriptions we have of it) 
seems to reproduce, on a smaller scale, the scenery of the table-lands of the Colorado. The 
climate of the Llano is very dry, and as its surface has an average elevation of nearly 4,000 
feet, the fall of its draining streams is very rapid; consequently, their channels are deeply 
cut and bounded by precipitous banks. 
The contrast of physical features presented by the plains east and west of the mountains is 
not of merely abstract interest. It involves all the economical differences between a nearly 
uninhabitable desert and perhaps the best agricultural region on the continent. To the 
geologist, the change from west to east is not so pleasant. Instead of the everywhere abound- 
ing exposures of the strata—as frequent and complete as though made expressly to facilitate 
his investigations—he here finds an unbroken sheet of soil, covering the underlying rocks as 
completely as the integuments cover the bones of a well-fed animal. The harder strata may 
occasionally be seen in the banks of the streams, but even there the softer intervals are repre- 
sented by smooth and grassy slopes. From this cause the determination of the details of the 
geological structure becomes a matter of extreme difficulty, and on the march one can hardly 
hope to do more than make a skeleton sketch of the formations over which he passes, and 
which for many miles may be concealed from his view. Fortunately the rocky foundations of 
the plains have never been broken up, and the regularity in the succession of the strata gives. 
to the necessarily interrupted observations along a single line of travel a value they would not 
_ otherwise have. Made under such circumstances, the subjoined sketch of the geology of the 
Santa Fé road must be brief and imperfect, leaving much blank space to be filled by future 
observations, and embracing views in regard to the superficial extent, thickness, and sub- 
ns of formations which « are only provisional, and liable to modification by the discovery 
