ENVIRONMENTAL AND HISTORICAL FACTORS. it 
rounding wash-deposits of the slopes. The studies now in progress suggest 
that the critical line between erosion (above) and deposition (below) is 
a function of climate. It is a fact that in many cases the upper edges 
of the slopes are being dissected. Here, however, the changes are more 
complicated than in the former case. 
I shall content myself with the presentation without discussion, of a 
bare list of the criteria I have found to be available in this special case 
to distinguish the two kinds of deposits. I shall only consider particu- 
lars in the physical characteristics and shall omit differences in chemical 
composition and fossil content, which have been discussed in the litera- 
ture of the subject. . 
I have been asked for a statement concerning the origin of “caliche,”’ 
in the light of recent studies in the vicinity of Tucson. Little can be 
added in the way of description to the excellent account of Dr. Blake.! 
He applies the name to the surface crust, mainly of lime carbonate, that 
develops under the conditions of mild aridity in Arizona and northern 
Mexico. I think that this definition should be followed, and as suggested 
by Lee the term ‘‘cement’’ be applied’ to the crystalline matrix of the 
desert fill, a crystalline, calcareous cement-conglomerate, often containing 
considerable amounts of selenite and hyalite, and a strongly gelatinizing 
silicate, probably a zeolite. 
Dr. Blake assigns the source of the caliche to the ground-water drawn 
upward to the surface by capillary action and evaporated there, finding 
support for this conclusion in the fact that the well waters in the vicinity 
of Tucson are charged with the mineral constituents of the caliche. The 
conclusions of Dr. Blake have been verified by Ransome’ for the vicinity 
of Bisbee, and by Lee* for certain deposits in the Salt River Valley. 
While it is not impossible that this explanation may fit certain cases 
where the water-level stands within a few feet of the surface, it certainly 
can not be applied to the Tucson region to meet whose conditions the 
hypothesis was framed. Here, where the caliche is thickest, the water- 
level is from 60 to 120 feet below the surface, and it is a well-known fact 
that in fine soils capillary action can only develop a head from 6 to 12 
feet. Furthermore, in all observed cases where the ground-water stands 
close enough to the surface to furnish a supply for evaporation, the sol- 
uble- alkali salts and not caliche are deposited. 
1 Blake, W. P.: The caliche of southern Arizona, etc., Am. Inst. Mng. Engs., XxxI, 
—220. : ; 
She W. T.: Underground waters of the Salt River Valley. Water Sup. Papers, 
Pe, INO: 136. ; ; 
Se B. ie The Geology and Ore Deposits of the Bisbee Quadrangle, Arizona. 
Prof. Paper 21, U.S. G.5., 74. 
Sree a Welt slic, TT. 

