42 
GEOLOGICAL REPORT-THIRTY-FIFTH PARALLEL. 
remnants of the upper strata are found only here and there in isolated bluffs. Thus we find 
the conditions on this side of the Anahuachian chain very similar to those on the east, where 
the Great Llano breaks off into isolated hills and then disappears altogether. On the western 
side, the transition is not quite so abrupt; the strata appear to have been denuded more from 
the surface downward, so as to thin out gradually without leaving long lines of bluffs or 
escarpments as on the Llano. The general limit of the formation is shown on the map, 
although there are doubtless many isolated hills of the Cretaceous scattered over the plain and 
not represented. 
Extent of the tdble-lancls west of the Sierra Madre. —This is a convenient point from which to 
take a general view of the broad expanse of the great plain that lies between the Sierra 
Madre and the mountains, which form the eastern rim of the Great Basin. From the Sierra 
Madre up to this place, the survey followed the eroded valleys of the streams, and the vision 
was hounded on both sides by their high and rocky hanks, composed not only of the edges of 
thick horizontal strata, hut often capped with the harder and more unyielding solid lava. The 
observer, as he passes westward from the mountains, is thus placed below the general level of 
the plateau, which does not become apparent to him unless he stands upon the top of the mesas 
and can thus cast the eye over the whole. The point already reached in the description is 
about half way between the Sierra Madre and the high mountains of San Francisco, and here, 
as we have seen, the upper strata of the plain are denuded and washed away, so that the banks 
of the streams are not so high, and the country appears more level or gently rolling. From 
this place the vision is unbounded towards the north, except by the horizon. The plain 
stretches far away, without any vestige of a mountain range. Indeed, it is the continuation of 
this plateau which rises upon the flanks of the Park and Wasatch mountains at the far north, 
and through which the waters of Grand and Green rivers cut their deep caiioned channels. 
Farther south, these streams unite to form the great Colorado, which is also found traversing 
this grand plateau. The course of this river is marked by one of the most stupendous valleys 
of erosion known to geologists. This is the great canon of which vague accounts have reached 
us from time to time, and which yet remain to gratify the sight of some future explorer. This 
great vertical cut through a vast thickness of horizontal strata must of necessity exhibit them 
under circumstances peculiarly favorable to the geologist, who can there observe their succession 
and thickness from the top to the bottom. It is believed, by those who are best qualified to 
judge, that this canon in some places is not less than two or three thousand feet deep, the 
sides being vertical. It is, therefore, probable that the edges of the Cretaceous, the underlying 
red clay and gypsum formation, the coal measures and the carboniferous limestone are exposed 
to view in a regular succession on the walls of this mighty chasm. Undoubtedly also the sum¬ 
mit is crowned with more modern strata, the Tertiary, and then again by the outpourings of 
lava from the adjoining volcanoes. What a mine of fossils from different chapters of the 
world’s history must be exposed here in readiness for the student! It has been generally sup¬ 
posed that this great canon was in granitic or eruptive rocks ; but this, I think, can scarcely be, 
although it is undoubtedly true that rocks of this character are exposed at the base of the 
bluffs. The approximate position of this canon is shown on Captain Whipple’s map. 
South of the line of survey the same plain extends until it reaches the Mogoyon mountains, 
which appear to be its southern and southwestern boundary. According to Mr. Marcou the 
strata dip away from this range, the heads of the beds being turned towards it. This dip is, 
however, very slight, seldom or never exceeding ten degrees, as will be seen by Mr. Marcou’s 
notes. 
Valley of the Colorado Chiquito. —The mineral characters of the strata exposed along the line, 
in the valley of the Colorado Chiquito, do not appear to differ materially from those already 
mentioned as characterizing the gypsum formation on the Atlantic slope. Gypsum, however, 
does not appear to have been found in abundance on this part of the line. The presence of 
thick beds of white and grey and bluish marls is mentioned by Mr. Marcou, and near Litho- 
