14 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
foot dune. We dug the trunk out to a depth of 14 feet. All four plants 
were from branches of the same trunk, the lowest branch arising about 
16 feet from the base of the dune; the main trunk and the branches bore 
marks of rosettes of leaves at intervals all the way to the lowest point 
reached. The trunk was thicker here, about 4 inches, than at any point 
above. The strata in the cut showed that the yucca once stood on the 
front slope of the dune. The trunk sloped in the direction in which the 
dune was moving. In the plain in front of the dunes were occasional low 
plants of the same species of yucca. Considering all the evidence, the 
conclusion is irresistible that the yucca originally grew on the plain, was 
engulfed by the sand, and gradually grew through each successive layer 
of sand that drifted over it until the summit of the dune was reached. In 
the vicinity, at the rear of the dune, were other long trunks partly denuded 
by the passing of the dune (plates 5 and 6). 
The greatest flow of air over the dunes is from the southwest, yet 
other winds are strong enough to complicate the movements of the dunes. 
The major motion appears to be in an easterly or northeasterly direction, 
and places are found where the eastern margin of the white sands have 
advanced a mile within 20 years, although it is not to be taken for 
granted that the entire mass has such absolute annual rate of movement. 
The occupancy of a portion of the soil by a dune changes its physical 
qualities and leaves behind it such infiltrated material that when left 
bare a different series of plants finds a foothold, as was found by numerous 
observations. | 
Analyses of Sands.—The following report on the gypsum sand from 
the White Sands of the Otero Basin and on sand from the Chihuahuan 
Desert has been made by Dr. William J. Gies: 
I present herewith the results of my chemical analyses of the two samples of 
sand expressed by you to me from Tucson, Arizona, on February 16 and received 
by me on February 24: 
SAMPLE I. LocaLity: Orrero Basin, New MExIco. 
General Description.—Color white to delicate cream, with occasional very 
minute black particles. There were also a few reddish and yellowish-red grains. 
Now and then red specks could be detected in the white grains. Glassy grains of silica 
were present. Nearly all the grains were very small, about the size of those in ordinary 
sea sand. A few larger masses were made up of many of the small grains cemented 
or fused together. These masses were more cream-colored than the small grains; 
some contained a dark nucleus. They varied in size from such as were only three 
or four times the bulk of the uniformly small grains to a few which were nearly as 
large asa pea. No special crystalline qualities were observed in any sample of the 
sand. The grains were angular or rounded by erosion. Fragments of elytra of 
beetles were detected and occasional pieces of hair and small splinters were also 
seen. 
Before subjecting the sand to anaylsis it was passed through a copper sieve the 
meshes of which were just large enough to permit the passage of the typical and 
uniformly sized grains. Only a few grams of material, consisting of the larger fused 
particles, elytra of beetles, hair, etc., were separated in this way from four kilos of 
