212 GEOLOGY. 
It is from the surface of this gravelly plain that the sand is derived ; it is swept from it by 
the strong winds, and travels along until the force of the wind is broken or changed by some 
natural obstacle. At such places the sand is dropped, and soon becomes heaped up into undu- 
lating ridges, which simulate the billows of the ocean by their wave-like contour. All the 
principal sand-hills appear to have accumulated upon the edge of this terrace ; and they do not 
extend beyond it. "Those near the road, between Cooke's Well and the Alamo, do not exceed 
sixty feet in height, and this is much greater than the average elevation. From the tops of 
these there are, however, other hills visible, two or three miles eastward, which appear much 
higher, and may be over one hundred feet high. 
I ascended the hills at a point a short distance from the Mezquite Well, and crossed over 
among them to their northern limits. At that place the width of the belt was less than half a 
mile. It is, however, wider further to the east, but probably does not exceed one mile at the 
widest part. "The hills are highest near the bank, and gradually thin down into a gentle slope 
towards the north. "The outline of a section of the belt, from north to south, may be illustrated 
by the figure; the direction of the wind and the progress of the sand being shown by the arrow. 
SECTION OF THE SAND-HILLS. 
The wave-like outline of the hills is apparent, and it will also be noticed that the windward 
slopes are long and gentle, compared with those turned from the wind. The latter are abrupt, 
and are generally inclined at an angle of thirty degrees, being ‘as steep as dry sand will lie. 
The relation of the sand-hills to the terraces is illustrated by a short section. P (See Sheet VIII.) 
When one penetrates among these round sand-hills, so that every other object is shut out from 
view, they seem like gigantic snow-drifts ; and the low, rustling sound, produced by the moving 
sand-grains, is very similar to that made by hard, dry snow, when driven before a high wind. 
The sand does not rise high in the air, but bounds along on the surface, or only a few inches 
above it, so that the drift is enveloped in a sheet or atmosphere of moving grains. They 
gradually rise the slope, and when they reach the highest point, they fall down the steep bank 
to the leeward. 
All these drifts and hills are covered with the most perfectly formed little waves and furrows, 
corresponding with the ripple-marks produced by water. These air ripple-marks are, however, 
more perfect and regular than those made by water, and they extend over large surfaces. l 
The sand that has driven over the bank, along the road, has so covered the terrace from view, 
that it is generally supposed that the hills are formed entirely of sand ; the fact that the sand is 
merely a coating or covering to the terrace has not been recognized. It is, however, clearly 
the case, for at many places the larrea is seen growing up through the sand, twenty feet 
above the road. This could not be without a foundation for the roots more firm than is furnished 
by the bare sand. 
There are but one or two points, in the whole distance between the Colorado and the Alamo 
Well, where the sand-hills present a formidable appearance ; or it seems possible that they can 
encroach on the road so as to render it impassable. The progress of the sand appears to be 
stopped by the vegetation, (Larrea Mexicana and mezquite,) and by the natural receptacle formed 
by the steep bank over which it falls. This bank acts as à complete bar to the further progress 
ofthe sand. It is more perfect in this respect than a wall-like barrier could be, for all the - 
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