278 WATER FOR DRINKING AND WASHING. Book II. 
until it reaches the surrounding level, and this embank- 
ment is surrounded with annular zones of vegetation, in 
which are seen in successive rows most of the plants that 
grow on the plain in groups. This phenomenon, there- 
fore, is no doubt attributable to the same causes that 
regulate the formation of the zones, and arises from the 
circumstance that the smallest variations of height in the 
soil cause differently mixed strata and other conditions of 
moisture to appear on the surface. These are phenomena 
showing some of the laws which regulate the geographical 
distribution of plants on the smallest scale, and the shallow 
basins of water, many being only a few paces in diameter, 
may be called lakes en miniature. 
Water is found in this desert — for such indeed the tract 
between the Arkansas and the Cimarron must be called — 
in a second form : in irregular holes in the sand, called by 
the waggoners " sand-pans." The reader may easily con- 
ceive that in both these natural reservoirs the fluid is 
neither very clear nor pure ; and I must remark, that in 
this journey, as well as on my subsequent travels through 
the interior of the continent, I speak of pure water as of 
exceptional occurrence. When I use the term water it 
generally designates a brackish mud, and for a long time 
I have drank water which was not clean enough to wash 
my face in. On these muddy pools in the desert between 
the Arkansas and the Cimarron, I saw on this journey the 
first wild ducks. They increased in numbers as we pro- 
ceeded westward. 
We travelled throughout the night, and the following 
morning encamped close to a pool of water. The wind 
blew cold from the north, and one of our drivers was 
suffering so much from rheumatism as to be quite disabled 
from service. I advised him to take a bath in the pool, 
