GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 65 
the upper layer is formed at the level where the water of the soil is vapor- 
ized faster than it can be supplied from below. 
The conditions of the upward movement of the water must vary 
greatly with the subjacent supply and with the nature of the formation. 
Thus at the site of the Laboratory at Tumamoc the conditions of flow 
and moisture from below upward are very different from those of the 
gravelly plains of Tucson. The basaltic rock appears as an almost 
impermeable barrier to the upward flow of solutions, except where there 
are structural planes or seams and cracks, and it is in precisely such 
places we find the calcareous deposits on Tumamoc Hill, where also it 
fills fissures in vein-like forms. In one or more such fissures the deposit 
is in considerable quantity. Near the top of the ridge, on the east side, 
a large crevice or vein-like space is filled with the calcareous deposit and, 
having the semblance of a mineral vein, has been dug into by prospectors 
in the hope of finding valuable ore. The deposit is in successive layers 
on the walls and is like some of the deposits of lime-onyx, and is possibly 
in this instance the result of a downward flow of calcareous water in a 
fissure, supplied from the formerly overlying tufaceous deposits. 
In ascending the north slope of the hill it may be noted that in places 
the otherwise loose blocks of black basalt are firmly cemented together 
with the lime deposit, so as to form a compact mass as if by the hands of 
a mason, as already described. 
A similar cementation of boulders is found at the tufa quarry on the 
west side of the hill. 
A chemical analysis of caliche, excluding the mechanically inclosed 
sand, shows the presence of: 
Calcium carbonate........ 12S Alumimum silicate: 2>,. 02. yy 
Magnesium carbonate.... 2.13 Antares urey.« (el ree eee 1.88 
PaAlcinmisilicate:.. 1.555%. Seay Misistirey aves leas ote ts toga 
The concentration of salts more soluble than lime carbonate by sur- 
face evaporation in the dry season, such as salt, sulphate of soda, and in 
some cases carbonate of soda, or ‘‘ black alkali,” are other examples of the 
same conditions of origin. In the rainy season these soluble salts are 
carried downward and disappear in the soil, reappearing as efflorescences 
at the surface when the rains cease and the dry air turns the direction of 
the movement of the solutions.* 
The general occurrence of travertine about the shores of ancient lakes, 
notably of Lake Cahuilla; at the sink of the Carson in Nevada, and at Lake 
Bonneville, Utah, taken in view of the evidences of an ancient submer- 
gence of the Tucson Valley, leads to the question whether or not the calt- 
che is a deposit from overladen calcareous waters during submergence. 

*I have elsewhere directed attention to the possible enrichment of the crop- 
pings of secondary ores in mineral veins by an analogous process of concentration 
under desert or desiccating conditions. 
