138 C. F. TOLMAN 
earlier portion of this time was spent in constant traveling over 
southern Arizona, and northwestern Mexico. In order that the 
value of the observations and deductions presented may be approxi- 
mated, it is frankly admitted that until recently attention was not 
directed to the broader features of desert physiography. The 
attitude of mind, bent on the study of the details of ore deposition, 
and therefore investigating minor features so important in that study, 
was carried over to the examination of the sedimentary deposits, 
and details of texture and structure, and their possible explanations 
were readily noticed. It is believed that such a study, undertaken 
systematically, however, will reveal the intimate relation between 
climate and deposition. 
PHYSIOGRAPHIC DIVISIONS OF ARIZONA 
Arizona has been divided into the plateau region, the range region, 
and the volcanic region by Gilbert, and later by Ransome into the 
plateau region, the mountain region, and the desert region.t A 
twofold division is preferred for the purposes of this paper, the 
separating line being the great Mogollon escarpment and its exten- 
sions, as shown on Robinson’s map of the Colorado Plateaus. The 
northern portion is designated as ‘“‘the mesa or plateau region,” and 
the broken area to the south the “bolson” country. The plateau 
region will not be under consideration, the student being referred to 
the unsurpassed descriptions of Dutton, and the recent work of 
Robinson and Lee. 
The generalization that this southern region is an intricately 
broken-down portion of the central plateau is remarkably true con- 
sidering the broadness of the statement, the regularity of the broken 
fault-blocks, however, is modified by the complicated post-Carbonif- 
erous intrusive masses. Further the form of the secondary hills is 
t Gilbert, ‘‘ Report on the Geology of New Mexico and Arizona,” U. S. Geol. and 
Geog. Survey of the tooth Mer., Vol. III; Ransome, ‘‘The Geology of the Globe 
Copper District,” Prof. Paper 12, U.S. G. S., pp. 14-16, with map. 
2 Robinson, ‘‘Tertiary Peneplains of the Plateau District,’ Am. Jour. Scz., 
August, 1907, p. 123. 
3 Dutton, ‘Tertiary History of the Grand Canyon Region,” Mon. II, U.S. G.S.; 
Robinson, op. cit., pp. 109-29; Lee, ‘Geology of the Lower Colorado River,” Bull. 
Geol. Soc. Am., Vol. 17, pp. 275-85. 
