Drainage Systems of New Mexico. — Tarr. 263 
plains even in places where the latter show no signs of rejuve- 
nation. The mountain streams flow through broad valleys of 
construction and narrow gorges of erosion with rapid slope 
very much as in the case of any Rocky mountain stream. The 
difficulties in the way of erosion in such places are great, yet 
they have been overcome and well established valleys have 
been cut. Undoubtedly the mountain erosion has been in 
progress with slight interruption since the mountains first 
appeared as such. 
After emerging from the mountains the Rio Grande flows 
upon a plateau, lava-capped in places, and almost everywhere 
composed of loose, partially consolidated. Tertiary fresh water 
deposits of cross-bedded sands and conglomerates. In the 
northern portion the river cuts a deep caiion through the 
lava and underlying sands ; but this canon character ceases to 
the south, and near the Texas line has entirely disappeared. 
Here the river flows near the top of the plateau in a broad 
valley a hundred feet or less below the level of the plateau. 
The Rio Grande floAvs directly southward from Colorado to 
Texas in a broad valley of construction between the broken 
ranges of mountains, and its tributaries have for their divides 
mountain ranges ; but in no case that I know do they cut 
across them. This is very difi'erent from what is generally 
noticed in mountainous regions. In Montana, for instance, 
almost all the large rivers cut directly across mountain ranges 
in actual erosion valleys. This is the case in the Madison, 
Jefferson, Gallatin, Missouri, Clark's Fork of the Columbia, 
and others. I am inclined to take this as evidence that at the 
time the mountains in New Mexico commenced to be crum- 
pled the territory was beneath the ocean and that the great 
plateau regions were not raised above sea level luitil the 
mountain folds had assumed their present form, although 
probably not their present elevation. The fact that the moun- 
tain erosion is relatively so much further advanced than the 
plateau erosion seems to point to the same conclusion. 
In the whole course of the Rio Grande from the northern to 
the southern boundary of New Mexico the river valley is for the 
most part carved out of soft unconsolidated sands and conglom- 
erates. Nearly all of these deposits are old lake or inland sea 
beds of late Tertiary age. My studies have not been made in 
sufficient detail to tell whether the sands were deposited in 
