64 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
CALICHE. 
The widespread calcareous formation below the surface of the soil 
in the Southwest, known as ca iche, is a variety of travertine (carbonate 
of lime) of terrigenous origin and deposit. The composition of caliche, 
its form and distribution are factors of prime importance in any discus- 
sion of the growth and distribution of plants in regions where it occurs. 
Its influences in the soil are not only chemical but mechan:cal, and it 
exercises an important influence in the distribution of surfaces and under- 
ground waters. 
Although the phenomena of distribution and the origin of caliche 
have been elsewhere described,* it seems appropriate in this place to 
review the principal facts relating to it, especially those which have a 
bearing upon plant-life. 
The broad valley of Tucson affords good opportunities for the study 
of the phenomena of caliche, but although the deposit is generally present 
except in the bottom-lands, it is usually covered from view by a foot or 
two of earth. A furrow can not be turned by a plow without revealing 
it, and if trees are to be planted, the hole must be deepened by a sharp 
pick or by blasting. Caliche forms practically a continuous sheet, a 
foot or two under the surface, from 3 to 15 feet or more in thickness, of 
travertine-like lime deposit, with the more dense and impervious layers 
at the top. This upper surface is comparatively smooth, though undu- 
lating, and often has knob-like excrescences. In fracture the upper or 
thicker crust exhibits fine lines of edges of layers or successive coats, 
along which separation may take place. There is also a rude columnar 
structure transverse to the layers. 
This hard upper crust, which seems like a layer of impervious cement, 
and which certainly retards the downward percolation of water, has, 
however, here and there minute perforations like pin-holes, at the top, 
which gradually enlarge downward and become lost in the more porous 
portions. These little holes are often occupied by the rootlets of plants. 
This deposition of lime-carbonate appears to be the result of the 
gradual upward percolation of the calcareous phreatic water supplied 
from the subterranean streams, induced largely by the excessive surface 
evaporation under continued desiccating conditions. Its absence in 
the soil at the immediate surface may be explained as due to the solvent 
action of meteoric water soaking downward, carrying the lime with it. 
Dr. B. E. Livingston in his discussion of calichet has suggested that 


*The Caliche of Southern Arizona, an example of deposition by the vadose 
circulation. Transactions American Institute of Mining Engineers, vol. XXxXI, pp. 
220-226, 1906. 
{The Relation of Desert Plants to Soil Moisture and Evaporation, by Burton 
Edward Livingston. Carnegie Institution of Washington Publication No. 50, 
August, 1906, p. 8. 
