ENVIRONMENTAL AND HISTORICAL FACTORS. 73 
Professor Forbes, of the Agricultural Experiment Station of the Uni- 
versity of Arizona, has kindly given me the following statement of his 
conception of the origin of the caliche: 
Caliche is a mixture of (probably) colloidal clay and carbonate (mainly) of lime. These 
two constituents, one in solution in the carbonated rain-water, and the other in suspension, 
are carried from the surface of the desert soils to the depth to which occasional rain pene- 
trates, which in this region varies from a fewinchesto3or4feet. At the general average 
level at which the wetted soil dries out through the desiccating action of an arid atmos- 
phere a more or less compact caliche stratum is formed. In situations where the soil 
surface is filled in from time to time new caliche strata are formed below each new soil 
level, the more recent formation therefore being above, the older below, in such sec- 
tions as are revealed by well-borings near Tucson. 
This explanation with some modifications will account for the occur- 
rences I have observed. Professor Forbes recognizes only the descending 
percolation of the water, but there is often a subsequent drawing to the 
surface by evaporation, whenever the drying out of a soil after a rainfall 
overtakes the downward percolation of the water, and taps the reservoir 
of moisture that is retained, even under arid conditions, just under the 
porous soil surface. To develop crusts there must be some ready supply 
of calcareous matter, and therefore no active underground drainage to 
remove the same. In the case of the Tumamoc Hills the source is the 
igneous rocks, especially the amygdaloidal cavities, as described later. 
In the vicinity of Tucson, the widely distributed Paleozoic limestones 
were the original source of much of the calcareous cement of the fill. 
As the gravel deposits are accumulated the caliche layers are built into 
them and slowly recrystallized by the percolation of the deeper waters. 
The development of the caliche is a rapid process, and in this it is in keeping 
with the acceleration of geologic action under aridity. In one place under 
observation the caliche was broken up and mixed with lime and brick 
left after building operations, and the mixture leveled and packed. No 
vegetation was allowed to grow and the ground was flooded frequently 
to keep the surface packed and lay the dust, etc. Within two years’ time 
there had developed 2 inches of typical caliche crust within half an inch 
of the surface. In other places, where the ground was not flooded, the 
new caliche is recognized with difficulty. 
The body of this article is divided into three divisions, viz, Topography, 
Geology, and Petrography. Prof. F. N. Guild’ published a microscopic 
description of the rocks of this area in 1905. He has kindly consented 
to examine some new material and revise his descriptions. His contri- 
bution appears under the head “ Petrography.” 
TOPOGRAPHY. 
The largest of the group of three hills already mentioned has been 
rechristened with its old Indian name, Tumamoc, by the staff of the 
Desert Laboratory. It was formerly known as Turtleback. Of the two 

*Guild, F. N.: Petrography of the Tucson Mountains, Am. Geol., Xx, 313-318. 
