10 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
in the interests of water development the evident impossibility of 
development in certain places rendered it impracticable to make 
extended geologic examinations where such examinations would have 
been of great scientific interest. For these reasons the fragmentary 
data here presented might perhaps be of little use if they related to a 
region fairly well known, but so little is known either of the geology 
or of the geography of this region that the information, confessedly 
incomplete, will, it is believed, be of value to many persons. 
The northern third of the region is covered by the early reconnais- 
sance maps of the United States Geological Survey, and the geographic 
features are well known. Little has been done, however, toward 
accurately mapping western Arizona south of the thirty-fifth par- 
allel. The maps heretofore published were found erroneous, espe- 
cially in the locations of mountain groups and lines of drainage. It 
is not claimed that the map accompanying this bulletin is errorless, 
but in the southern part, where previously published maps have 
been most incorrect, this map is believed to be measurably accurate 
owing to data acquired by recent railway surveys and furnished for 
use in this bulletin by W. A. Drake, engineer of the Santa Fe Rail- 
way system. 
The geologic formations were mapped where practicable, and their 
general distribution over the greater part of the region was deter- 
mined. Data concerning their character and occurrence were col- 
lected as far as time would allow, and special attention was given to 
the recent geologic history. 
During the investigation it became evident that changes of a physi- 
ographic nature have occurred in recent geologic times, which have 
produced notable results. Changes in surface elevation and configu- 
ration and in climatic conditions have resulted in periods of aggrada- 
tion and periods of degradation during which nearly the entire region 
was affected. The evidences of these changes are found in such 
phenomina as recently formed canyons, filled valleys, aggraded 
plains, and the deflection of streams. 
The chief economic results of the examination have to do with the 
development of both surface and underground waters. Attention 
is also called to the mining districts of the region, and certain possi- 
bilities of development of the mineral resources are pointed out. 
Summary of previous knowledge. — Little information has hitherto 
been published regarding the geology of this region. In 1856 Jules 
Marcou's notes were published in the Pacific Railroad Reports, a in 
which he briefly describes the geologic features along the route trav- 
ersed, including the igneous and granitic rocks of the Truxton Pla- 
teau, the detrital filling of the Big Sandy Valley, and the gneiss and 
o Marcou, Jules, Pacific Railroad reports, vol. 3, 1856, p. 155. 
