Re OR Pe da u MT ET T مت‎ rA 
SAND-HILLS IN THEIR RELATION TO A RAILROAD. 243 
that pours over its edge is protected from the further action of the wind, which is allowed to 
preserve its general direction, and passes on without being thrown into violent eddies, as would 
be the case if its progress was impeded by a wall, or any prominent object. 
The movements of the sand in the air are precisely similar to those that take place when it is 
immersed in the more dense fluid, water. The progress of the grains along the surface of the 
plain, and their final rest at the edge of the bank, is precisely similar to the transportation of 
sand by a stream, and its deposition, in the form of a bank, whenever the current enters deep 
water. In water, little eddies and back currents are produced by a projecting rock, or root, 
acting as a barrier to the current, and drift-sand accumulates on the lower side of such obstacles. 
So, in air, wherever a slight obstacle, such as a bush, or boulder, stands on the plain, exposed 
to the wind, the driving-sand accumulates on the lee side, as in the figure. 
مد بر 
` We have thus considered the phenomena, and the mode of progress of the principal sand-hills 
of the Desert, and the conclusions to which we may arrive may be stated as follows: 
1. The sand is derived from the surface of the upper gravelly plain of the Desert by the 
, continued action of the northerly winds. E 
2. These sands have accumulated along the margin of this plain, and form a belt less than a 
, mile in width, and about twenty miles in length. 
3. The hills seldom reach a height of sixty feet; and most of the high hills, heretofore 
supposed to be composed entirely of sand, are underlaid by a high bank of clay and gravel, and 
the sand is a mere covering. : 
4. That the terrace, forming the margin of the plain on which the.sand rests, acts as an 
effectual bar to its further progress. i 
In view of these- faets, I see no reason to regard the Sand-Hills as a serious obstacle to the 
construction of a railroad. They occupy so small a part of the wide area of the Desert, that, if 
necessary, they can be avoided. The road could be safely built on the hard clay surface of the 
first or second terrace, carrying it several miles south of the sand ; or, if it is desirable to run in 
à more direct line, from the Gila to the Pass of San Bernardino, the road can be made on the 
Upper plain, far in the rear of the sand, and on its windward side. It is not necessary to carry 
the road across the Sand-Hill range. The hills have a direction that would be nearly parallel to 
the course of a road. 
There is a thin layer of sand on the bank above the Alamo Well, but it does not form a drift 
or hill of such a magnitude as to render it worthy.of particular notice, Sand is again met in 
similar thin Sheets, varying from three inches to a foot in depth, in the vicinity of the Lagoon. 
It fills up the little crevices and little irregularities in the surface of the clay, and is readily 
shifted about by the winds. Several drifts were also passed between the emigrant trail and the 
Salt Creek. They are long and narrow, and do not present any formidable appearance ; they 
are not deep ; and the wagons of the train crossed them without much difficulty. They form 
long and narrow strips or belts of sand, which are only a few inches deep for the greater part of 
their extent; but in one or two of the highest points, their depth may be ten feet. At Deep 
Well the sand is piled up into a range of low hills, not over twenty-five feet in height. They 
