276 The American Geologist. November, 1905 
great stake plains of Texas are made up largely of Creta- 
ceous sediments which have a more or less regular dip from 
the frontal ranges of the Rocky mountains to the gulf of 
Mexico. Toward the New Mexico border of this plain the 
strata have stronger dip and erosion has exposed the edges 
-of the strata along the Rocky mountain front. The Creta- 
ceous terranes are covered with a mantle of Tertiary gravels 
■derived from the mountain front. 
Dr. Keyes says : "The Las Vegas plateau, Llano Esta- 
•cada, the bolson plains of New Mexico, and some of the 
broken plains of eastern Arizona seem to belong genetically 
together;" and he further says: "When the general bow- 
ing up of the region took place in Tertiary times, the great 
plain formed was partly a pleneplain of destructional land 
origin and partly a constructional plain of marine origin." 
From this it would appear that the bolson plains of New 
Mexico, as he describes them, are remnants of this old 
peneplain, and that the mountain blocks of the plateau of 
New Mexico were formed subsequent to the peneplanation 
of the Cretaceous and lateral beds. It is not possible at the 
present writing to present the data to show that the struc- 
ture of the mountains of New Mexico will not sustain this 
position. It is the writer's desire in this discussion, to con- 
fine attention wholly to the valley forms. 
Dr. Keyes says : "That the old bolson plain in the Rio 
Grande valley is at present about 1,500 feet above the 
river" and he refers to the Colorado river as being "a mile 
.deep in its canyon" below the surface of the great plain 
which he has constructed in his hypothesis. 
During a residence of four years in the Rio Grande 
valley, accompanied with considerable field work, it has not 
teen my pleasure to see a single remnant of the old plain 
to which reference is made. The valley of the Rio Grande 
through New Mexico has an extremely complex and varied 
form, and history. I desire at the present writing to call 
attention to only a few points in its history bearing upon 
the particular discussion in hand, and what is said with 
reference to the valley includes only that section which lies 
within the territory of New Mexico. The river is at Several 
points, notably at White Rock canyon, at Elephant buttes. 
