Bolson Plains of the Southwest —Tight. 277 
and at El Paso, cutting through a rock gorge upon a rock 
floor, but throughout most of its course it is meandering 
over a broad flood plain in a still broader shallow trough, 
the latter cut some 200 or 300 feet below the surface of 
the broad sheet of plain deposits locally known as mesas. 
The larger structural valley which is followed by the Rio 
Grande is undoubtedly of very complex origin and no gen- 
eral description would be adequate for any particular sec- 
tion of the river. From the limited amount of data in hand 
it would appear that in some sections the river is following 
the line of a great fault zone and in other sections it is 
apparently following along the axis of an immense anticline, 
which has been very deeply and broadly eroded. The great 
structural valley presents an average width of fifteen to 
twenty miles, measured across the surface of the great mesa 
plain. 
If we examine into the structure of the mesas border- 
ing the river, as presented by well sections and deeply cut 
arroyos, we find that it is made up wholly of sands, gravels, 
and clays of fluviatile origin. The materials composing the 
mesas have been largely derived from the lateral mountains. 
The depth of this mesa deposit in the old structural valley 
is not definitely known. A well over seven hundred feet 
deep at Albuquerque did not reveal the rock and as the top 
of this well is about 250 feet below the surface of the mesa 
plain and near the central portion. of the great valley, we 
can see that the great structural trough has been filled by 
the mesa deposits to the depth of probably much more than 
a thousand feet. It is evident from many topographic 
features that the river once meandered over the upper sur- 
face of this mesa plain at least 250 feet above its present 
level. At about that time in the history of New Mexico 
there occurred a more or less general extrusion of basaltic 
lavas over many areas. At least two of these lava overflows 
reached down into the valley of the Rio Grande and attained 
such magnitude as to produce profound changes in the 
course of the river. The first of these which I will mention 
is the great lava flow in northern Xew Mexico which dam- 
med the course of the Rio Grande above the White Rock 
canyon, and the second, trie threat lava' How south of San 
