GEOLOGY OF THE WHITE SANDS OF NEW MEXICO 123 
About one mile from the Mal Pais spring above mentioned 
is a small salt lake which has furnished the salt for ranches for 
a radius of many miles during the historic period and at our visit 
the surface was covered to the depth of an inch or so with pure 
crystalline chloride of sodium. Still west and forming the west- 
ern limit of the visible saline beds, is a drainage arroyo whose 
source seems to be in the red beds that emerge west of the Oscuro 
Mountains and conveys their saline water to the basin of the 
sands. Along the course of this arroyo are numerous salinas 
and alkali flats and these gradually broaden to form what may 
be described as one vast alkali and salt plain where brine stands 
for part of the year. Other arroyos come in from the west in 
some of which, even at the time of our visit, was a considerable 
quantity of flowing water which is a strong brine unfit for cattle 
even when accustomed to drink from the saline springs which 
unwonted animals will reject. Where these arroyos enter the 
salt lake and along the shores of the lake are bluffs of erosion 
some of which are over twenty feet high. In these exposures we 
encounter the red bed formation with its marls and gypsum 
deposits. Large quantities of pure crystalline gypsum are here 
exposed and the marls are alkaline and saline. We have there- 
fore local proof, as well as the most conclusive evidence from 
the environs, that the whole of the plain is in or near the horizon 
of gypsum and salt that separates the lower from the middle 
member of the red or saline series. 
In the salt flats the ribs of gypsum rise in successive ridges, and 
the action of the elements soon breaks up the exposed crystals 
into small grains which are carried by the winds hither and yon. 
This characteristic of the salinas accounts for the most curious 
and notable of the many peculiarities of these plains, namely the 
white sands. These have been attributed to the action of springs 
and the material has been supposed to have crystallized. from 
solution. It has been suggested that the sands have been col- 
lected by floods, but a short examination shows that these great 
drifts are simply sand dunes collected from the gypsum sand 
formed as above stated on the surfaces of the lakes. The salt 
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