8 GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
found to be about 4,400 or 4,500 feet, the camp on the east side of the bluff being 4,128 feet, and 
on the west 4,207 feet. We may regard 4,500 feet as about the mean elevation of this great 
plain. The mean elevation further south, according to the measurements made under the direc- 
tion of Captain Pope, along the route near the 32d parallel, is 4,500 feet ;! but this result, 
although the same as we have deduced from Lieutenant Whipple's observations, is probably too 
high, for we have reason to believe that the Llano decreases in altitude towards the south. 
Captain Pope's observations were made by the theodolite, and the altitudes are possibly too 
high ; the results obtained by the barometer are deemed the most reliable. 
Plain between the Sandia and Santa F6 mountains, and the Sierra Madre.—This plain, or 
valley, between the mountains east of the Rio Grande, and the Sierra Madre on the west, has a 
width of nearly one hundred miles. The eastern side of the valley is traversed by the Rio Grande 
and the Puerco. The latter is a much smaller stream, but runs parallel with the first, and unites 
with it several miles below the line of survey. From the Puerco the ascent to the Sierra Madre 
is gradual, the grade being 23 feet to the mile. The surface of the plain is, therefore, not level, 
but has the character of a slope flanking the Sierra Madre, and is not unlike that east of the Santa 
Fé mountains. The topography of the plain is also similar ; the streams cut grooves, or channels, 
below the general surface, and are bounded by bluff banks of horizontal strata. "The barometric 
profile follows the course of the Zio San José, which, rising in the Sierra Madre, flows west to 
the Rio Puerco. It is interesting to note, that the long slope of this wide valley between the 
mountains is turned towards the east, and in this respect it resembles the slope from the Santa 
Fé mountains to the Mississippi. The average elevation of this gently ascending plain is about 
6,000 feet, and it probably connects further north with the broad area of table-lands traversed by 
the tributaries of the Colorado river. 
Plain or table-land west of the Sierra Madre.—After passing the range of the Sierra Madre, an 
immense expanse of table-land is spread out before the explorer. From this range to the volcanic 
cone of San Francisco, two hundred and fifty miles distant, there is not a single mountain ridge 
or sudden swell of the surface to break its monotony. It is a region of horizontally stratified rocks, 
cut and eroded by streams as on the eastern or Mississippi slope. The descent here, however, 
is towards the west, and is very gentle. The same or similar rock-formations are found on this 
side of the great dividing range, and in the same horizontal position: the topography is conse- 
quently similar. The erosions and bluffs produced by the head-waters and tributaries of the 
Canadian, on the elevated plateau of the Llano, find their counterpart on this side of the mount- 
ains in the Colorado Chiquito and its tributaries. As in approaching the mountains on the 
east, the survey followed the pathway thus cut out by the streams ; so in descending on the 
west, the expedition followed similar paths, cut by the streams which flow into the Pacific. 
The extent of this wide area of table-land is not yet accurately known. It was seen stretching 
out indefinitely towards the north, and doubtless is continuous, and of the same character, as 
far as the head-waters of the Great Colorado, and Grand and Green rivers. The great cañon 
of the Colorado is also in this plain, and on the south it appears to extend to the base of the 
Mogoyon mountains. For a time after quitting the Sierra Madre, the line of survey passed 
over the upper strata of the plain; then reaching the waters of the Colorado Chiquito, this 
valley was followed to near the base of the volcano, where the valley was left, and the ascent of 
this elevation was commenced. Thus the barometric profile represents a continuous and nearly 
uniform descent westward for a distance of about two hundred miles. The slope of the surface 
on this western side of the Sierra Madre is, therefore, longer and more gradual than that on its 
eastern flank. The barometric profile in this case, as in the eastern slope, does not represent the 
stair-like descents from the higher levels of the plain to those below, for the simple reason that 
they do not appear in the vaHeys of the streams. In the profile constructed for the geology, how- 
ever, this important topographical feature is represented. The lowest point of this slope reached 
! See Report on the Geology of the route near the 32d parallel, p. 9, 4to. Washington: 1856.—[P. R. R. Ex. & Survey. | 
