i66 The American Geologist. ^^rch, looi. 
atively modern date geologically are more abundant in the 
southwestern portion of Arizona than in the central and eastern 
portions, but there are considerable areas of rhyolyte in the 
Chiricahuas and of volcanic tuffs and lavas at Tucson, and in 
the valley of the Hassayampa south of Prescott. 
Mesozoic. Formations of the Mesozoic are not absent in 
southern Arizona. The massive red sand-stones north of 
Phcenix, of Tempe and Mesa in the Salt river valley are re- 
ferred to the Trias. So also the extensively developed red beds 
of the southern side of the Bradshaws at Castle creek. East 
of Tucson near Vails and Pentano, stratified sandstones and 
shales are probably Mesozoic. A wide area of disturbed red sand 
stones and shales between Oracle and the Gila at old Camp 
Grant and apparently unconformable to the uplifted conglom- 
erate, quartzyte and limestone are probably Mesozoic. 
There are evidences of the presence of Mesozoic sandstones 
in the Sulphur Spring valley. The Trias is largely represented 
south of the international boundary line in Sonora. But the 
identification in all these localities rests upon stratigraphical 
and lithological characters rather than upon fossils. 
The recent discoveries of remains of Elephas and of the 
Mastodon at several distant points, and of the horn cores of 
a giant form of Bos all indicate former conditions of greater 
precipitation and of more abundant vegetation than now exist. 
Pleistocene Lakes. We also have evidences of extensive 
interior lakes in the later Tertiary or in the Pleistocene period. 
The valley of the San Pedro exhibits a great thickness of hori- 
zontal lacustrine clays, generally of red color, extending from 
the Mexican border northward to and beyond Benson on the 
Southern Pacific R. R., where they are cut through by the river 
to a depth of 600 feet or more. The northern barrier of this 
lake was probably in the narrow valley between Benson and 
Dudleyville. By the cutting away of this barrier the lake has 
been drained. For this ancient lake I have proposed the name 
Quiberi, the ancient Indian name of the San Pedro valley and 
river. 
The great Sulphur Spring valley between the Dragoon 
mountains on the west and the Chiricahua mountains on the 
east is also regarded as the dried up bed of an ancient lake of 
great extent. 
