GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 67 
their sustenance proceeded together, and were due to increasing heat and 
dryness rather than to increasing cold. 
We have ample evidence that in the Cretaceous era conditions in 
Arizona were favorable to forest growth and luxuriant vegetation. The 
coal-beds of Deer Creek near Saddle Mountain in Pinal County, described 
by Emerson, reveal such conditions. 
Quantities of silicified tree-trunks in the vicinity of Yuma and the 
prostrate forms of giant trees turned to stone in the Petrified Forest 
Park bear eloquent testimony to such forest growths and to destructive 
climatic changes in Tertiary time. 
More recent evidence is found in springs surrounded by relics of vege- 
tation, such, for example, as Andrade’s Spring east of Tucson and on the 
right bank of Davidson’s Canyon, where there is a thick accumulation of 
sphagnum with stumps of trees and, at the bottom, teeth of the mastodon. 
The former existence in Arizona of a species of Bos of unusual size is 
shown by the discovery of enormous horn-cores in the gravels of the sec- 
ondary or derivative slopes of the Santa Ritas at Greaterville. 
SUBMERGENCE AND ELEVATION. 
In addition to other evidences of change of level of the Tucson region 
in comparatively recent geologic times, we have the ancient Lake Qui- 
buris, already described, which occupied the adjoining valley on the east. 
The vast accumulation in this valley of lacustrine clays and silt, now 
exposed to view on each side by erosion, bears good evidence of the long 
duration of the submergence of the valley and of the height of the water 
at about 4,000 feet, or nearly the height of the divide between the Tuc- 
son Valley and San Pedro, corresponding to the indications of the ancient 
level about Tucson. Without assigning this figure as the limit of the 
depression or of the total elevation of the region of the southwest, the 
comparatively general and uniform altitude of the detrital slopes favors 
the view that the sea-level rested for a long period at about that altitude. 
Upon this assumption it becomes interesting to note what the form 
of the coast-line must have been during the period of depression, and to 
illustrate it the accompanying sketch-map has been prepared (plate 52).* 
It presents the land areas of the southwest, including Arizona and 
the southern portion of California, which rise above the contour line of 
4,000 feet. The areas of less altitude are represented as under water. 
Without claiming absolute accuracy in delineation, the map serves 
to give a general idea of the coast-line before the Pliocene uplift and to 
help elucidate some phenomena of the geographical distribution of the 
plants of the area and of the probable climatic conditions in Tertiary time. 

*This was exhibited by the author at a meeting of the Cosmos Club of Tucson, 
in May, 1906. 
