66 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
It is also to be noted that the absence of caliche from the alluvial deposits 
tends to support the view of deposition from overlying water. Further 
evidence and the careful study of all the phenomena are desirable. 
CHANGE OF CLIMATE. 
A change of the climatic conditions throughout the Southwest, and 
especially in the semi-desert region of Arizona and New Mexico, is 
marked everywhere by the evidence of a much heavier rainfall than we 
now have. River valleys in many cases show only dry gravelly or sandy 
beds which evidently were formerly occupied by continuous streams. 
The floods that once carved their way across the slopes or over the plains 
are no longer seen, at least not in the same volume as in former time. 
Even existing streams do not reach in times of great flood their former 
volume and carrying capacity. All tell of diminished volume, whether 
in the desert regions or in the regions of abundant plant-growth. 
We may believe that the cause is extraterrestrial and cosmic, and a 
part of the great era of climatic changes giving to the earth the glacial 
era, and its gradual decay. We may believe that the era of greatest pre- 
cipitation in the Southwest and elsewhere was coincident with the widest 
extension of the glaciers and that while the higher mountains were being 
loaded with snow, the lower slopes were deluged with rain or watered 
freely by the melting snows and enjoyed a verdure no longer possible. 
The gradual desiccation of Arizona and other regions may be regarded 
as synchronous with the gradual disappearance of glaciers, a condition 
now in progress, as shown by the retrocession of glaciers still in existence, 
even in the Sierra Nevada of California, where only remnants remain of 
the once mighty sheets of ice which covered that region. 
EXTINCTION OF THE GREAT MAMMALS. 
The vast extent of the ancient detrital deposits anterior to the uplift 
indicate a much greater rainfall than we now have, and this greater pre- 
cipitation must have continued during and possibly long after the uplift, 
and have exerted a great influence upon the nature and distribution of 
vegetation. 
The fact of the existence and wide geographical range in Arizona of 
the great mammals, the mammoth and the mastodon, shows a very 
different condition of vegetation up to comparatively recent geologic 
time. The extinction of these giant herbivores may be best explained 
upon the theory of the desiccation of the region rather than by a change 
of temperature or increasing cold, as apparently was the case in Siberia, 
and may have been in the glaciated regions of California. A great change 
in the rainfall and the drying up of the slopes and mesas of Arizona must 
of necessity have caused a great change in the growth of plants, involving 
their destruction over great areas. It would appear that the extinction 
of the giant mammals and the disappearance of suitable vegetation for 
