282 The American Geologist. 
November, 1905 
in thickness. In many cases the valleys were so completely 
filled that the detrital plains of the neighboring valleys were 
united, giving the appearance of extensive plains with iso- 
lated islands or bordering ridges. 
Where the How of water in the larger drainage axis 
like the Rio Grande was sufficient, the river contributed 
largely in the process of aggradation and plains building. 
\t or near the time of maximum aggradation, in the 
New Mexico region, at least, occurred the period of great 
basaltic lava flows. These were so distributed that in some 
cases, the lava flowed on to the bolson plains of the great 
isolated valleys or into the great plains bordering the Rio 
Grande and other streams diverting their courses. This 
great period of subsidence and aggradation is most striking- 
ly shown in portions of southern Arizona, where Dr. W. T. 
Lee of the Geological Survey, has told me that the fluviatile 
deposits following the great eroded channelways of the Col- 
orado and other streams extend to several hundred feet be- 
low sea level as is abundantly attested by well data. 
While it does not seem at all necessary to postulate the 
great deformations of the land to account for transitions 
from conditions of degradation to those of aggradaation, as 
in many cases variations of climatic conditions including 
precipitation and so on, may be sufficient causes, yet where 
it is known that the plain of degradation extends below the 
plain of the present marine base level, a difference in alti- 
tude must be assumed. Dr. D. W. Johnson in an extensive 
article on the High Plains and Their Utilization, published 
in the 21st annual report of the United States Geological 
Survey, Hydrographic Division, has described with much 
detail the method of the formation of the frontal aprons bor- 
dering the mountain areas in the semi arid regions with 
special reference to the great sheet of Tertiary gravels 
which are spread out over the high plains region, including 
the Llano Estacado. He presents a diagramatic section on 
page 729 of that report showing the relation of Tertiary 
gravels to the underlying Cretaceous over the Stake Plains 
plateau, and he rightfully, I believe, attributes their origin 
to the frontal ranges of the Rockies. His description of the 
method of formation and structure of the great Tertiarv 
