STRUCTURE OF THE PLAINS. 103 
coating of grass of the plains, present to the physical aspects of the western half of the great 
plateau, is most striking; and since the geological structure is nearly the same in both, the 
origin of this contrast forms an interesting subject of inquiry. I think these differences may 
be referred to the combined action of several distinct causes, mainly geological, but in part 
atmospheric. Of these one is to be found in the nearly uniform easterly dip of the rocks, which 
prevails over so much of this region, forming a grade of sufficient slope to carry off the drainage 
from the mountains freely but gently, without any such headlong descent as that from the 
western edge of the table lands into the trough of the lower Colorado. From this gradual 
descent of the surface the line of summits bounding the valleys runs parallel in elevation with 
the beds of the draining streams. Hence the valleys scarcely increase in depth though they 
grow constantly broader, and ultimately, by elimination of the dividing ridges, occupy the 
greater part of the surface.* 
Another and perhaps equally efficient cause of the topographical features of the plains is the 
more copious and general rain which falls upon them. The legitimate effect of this agent is to 
widen the valleys by washing down their sides, to round off and wear down the hills, to diminish 
all irregularities, in short, to accomplish its mission as the greatest of levellers. 
Indirectly the same effect is produced by the yrowth of vegetation formed by the moisture. 
This also has a tendency, as is known, to round over and soften down all asperities of the 
surface. It is true that the more elevated portions of the prairies are very dry as compared 
with the low humid banks of the Mississippi, but none of the area traversed by the Santa Fé 
road at all deserves the name of desert, or is comparable in dryness and sterility with the 
cafioned country west of the mountains. A thick mat of grass covers the ground, which could 
sustain an immense amount of animal life, an accurate index of the quantity of precipitated 
moisture. It should also be said that in the higher and drier parts of the plains, and there 
only, the streams traversing them flow through excavated cafions. 
It will have been seen in the sketch of the general structure of the table-lands given in 
Chapter V, that the plateau, now cut by the great cafions of the Colorado and its tributaries, 
had originally but a moderate descent from the base of the Rocky mountains to the great bend 
of the Colorado, but near that point was a fall of several thousand feet. The force of a running 
stream is constantly expended in establishing an equilibrium in all parts of its descent, and 
producing a uniform grade in its bed from mouth to source, and the Colorado, in the lapse of 
ages, has done much towards the accomplishment of its task, and has distributed the great fall 
at the Black mountains along several hundred miles of its course. 
* There are some — oer facts which have a bearing on this subject, perhaps no less important than those cited 
above, but they re nd extensive observation for their complete any] 
These are, he the thdieiitous that the country lying ‘between the Rocky Mountains and the Mississippi has occupied a lower 
level, and has been oftener and longer submerged during the Tertiary and Post Tertiary ages, than the highest plateau west 
of the Rio Gran 
Without come: the facts which teach this, since they have been enumerated in the preceeding pages, we may say that 
such a conclusion is apparently inevitable from them. 
The effect of the submergence of this region by fresh or salt water bering necessarily be, not only to forbid all erosion, but 
to deposit sediments, by which former inequalities would be, to a degree at least, filled u p and and the su made 
smooth and even. 
The second fact in this category is, that a large part of the prairie country has been swept by drift currents, or drift agents, 
whatever their nature, and by their action the rocks have been broadly and uniformly planed down, and the depression filled 
with the debris. 
West of the Rio Grande the drift action has never been in operation, but the erosion of the surface has been confined to the 
lines of drainage. 
During the Tertiary period that region was, as a whole, elevated above the sea level, and only locally covered by fresh water 
lakes. Since the Tertiary epoch it has apparently always held its present character : a high plateau, perhaps the highest of 
equal area in the world; probably the oldest portion of the earth’s present surface, at least one that has longest exhibited its 
present conditions; one that has been longest exposed to the influence of agents now in action, and hence bearing the most 
deeply inscribed records of their power. 
