98 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
sand-dunes the substratum is in constant motion of greater or less rapid- 
ity, lacks a suitable water-supply, and may be devoid of other nutritive 
material. Even if the dune areas are supplied with water in proper 
quantities, the peculiar character and movements of the substratum 
result in some striking forms of vegetation. 
A combination of the above mechanical and physical conditions of 
the soil and of the presence of harmful substances is offered by the white 
sands in the Otero basin in New Mexico, of which a more detailed descrip- 
tion is given on pages 11 to 15. 
The sand-dunes in this district consist chiefly of gypsum, the principal 
remaining constituents being silicates and calcium chloride in the propor- 
tions of 3 per cent and 1 per cent respectively. The gypsum is slowly 
soluble in cold water and retains the greater part of the water which 
falls upon it. Consequently the dunes are really moist hillocks of a 
granular structure, the surface layers of which are dried out by the heat 
of the sun to a depth of a few inches. The dried layer is constantly 
drifted by the wind, and the exposed layers are dried in turn, so that the 
progressive action of sand-dunes is manifested. The underlying layers 
at some depth often become solidified and stratified, but are easily broken 
up when exposed totheactionofthesunand wind. Themoistureincluded 
is sufficient for a number of species of plants, but the mineral substances 
in solution make it possible for only those forms which are adapted 
to an alkaline substratum to gain a foothold. The White Sands absorb 
the entire precipitation and give rise to no distinct streams, but occasional 
small pools or tanks of water highly charged with calcium salts are to be 
found in areas among the dunes. In western Australia extensive areas of 
gypsum desert are to be found which, it is reported, form a distinct harder 
surface crust instead of a granular layer, as in the instance described above, 
but no exact analyses of the substratum are at hand. 
A number of districts in America show inclosed pockets or basins, 
forming the extreme lower depressions of ancient lake and river beds, 
in which the soil is highly charged with salts, the most of which is sodium 
chloride. Examples of this character are offered by the region around 
Great Salt Lake, Utah, and by the Salton District in the Colorado Desert 
of southern California. The characteristic vegetation in both instances 
is composed of species showing halophytic adaptations resembling those 
found near the seashore. 
Limited areas in various regions show soils impregnated with sodium 
sulphate, sodium carbonate, potassium sulphate, sodium phosphate, 
sodium nitrate, calcium sulphate, calcium chloride, magnesium chloride, 
and magnesium sulphate. In agricultural operations two types of such 
soils are recognized, namely, whtte alkalt and black alkalis. 
