80 GEOLOGICAL REPORT-—THIRTY-FIFTH PARALLEL. 
route. The Llano is doubtless continuous southeasterly from these localities to the vicinity of 
Austin and San Antonio, where it forms bluffs filled with cretaceous fossils already described by 
Ferdinand Roemer. There is thus little doubt that the strata of the Llano Estacado and the 
other plateaux along the route, resting upon the gypsum formation, are of cretaceous age, sur- 
mounted, perhaps, in places by more recent deposites. 
Although the strata of the plateau beyond the Sierra Madre were not identified by fossils, 
if we except the rolled specimens of Gryphæa, we are led to regard them as of the age of the 
Llano, they being similar in their mineral characters, and probably connected with the creta- 
ceous deposites of the valley of the Rio Grande, through Campbell’s Pass. The altitude of the 
plateau and the underlying red strata containing gypsum, as on the east side of the mountains, 
leave little doubt of their contemporaneous origin; moreover, cretaceous fossils have been found 
in the northwestern parts of this extended area of horizontal strata.? 
As the strata which bear the fossils are conformable with the red marls, clays and sandstones 
below, which are without fossils, and consequently of undetermined age, and as there is no well- 
marked line of separation between them, we cannot decide upon the thickness which the Creta- 
ceous attains. It may include the gypseous strata, or at least the upper part of the series. 
But as this gypsum formation is separately described, the present descriptions will refer only to 
the lighter-colored strata which overlie it, and which form the upper part of the plateau of the 
Llano. The extent of these strata will be best seen on the map and section, where they are 
colored green, and thus separated from the red, which indicates the gypsum formation below. 
The Cretaceous is made coincident with the bluff margins of the Llano, and other plateaux, 
and thus may be said to extend along the route from the vicinity of the meridian of 101° to 
110°. It also appears in isolated patches or remnants of the table-lands, forming mounds or 
columns, as at Camp 31 and Antelope hills, and other points east of the bluff margin of the 
Llano. 
As the lithological characters of these strata have already been presented in detail in the 
descriptions of the geology along the route, they need not be repeated here. The strata are 
principally white or grey, and highly calcareous, and are sometimes intercalated, as at Pyramid 
mountain, with grey or blue marl or clay. There are few or no specimens in the collection, but 
the descriptions lead one to conclude that the strata are very uniform in their appearance along 
the route. The specimens which Captain Pope collected on the Llano, further south, are chiefly 
light-colored, whitish, or grey calcareous sandstone, and a buff or yellowish limestone, earthy 
and amorphous. 
The light-colored strata of the Llano do not appear, from the descriptions, to attain a thick- 
ness of over 500 or 600 feet at any point, and generally the red strata are described as extending 
half-way, or more, up the bluffs, which are from 300 to 600 feet high; and this leads to the 
conclusion that the white strata are generally less than 300 feet thick, but they have been 
observed in some localities to form nearly the entire bluffs, from top to bottom. The section of 
Pyramid mountain, given in Chapter II, shows the relation between the red and the white strata 
at that place, and presents the line of division, not in the centre, but nearly two-thirds of the 
distance up from the base. No observations regarding the thickness of each series of beds are 
given by Mr. Marcou, but the whole is stated to be 500 feet. The thickness of the white strata, 
doubtless, varies in different places, the red color of the strata being produced by decompositions 
since the original deposition, and, of course, not being confined to particular beds, but extend- 
ing higher or lower according to circumstances. 
The extensive and nearly level plateaux which these strata form, sufficiently show their hor- 
izontal position. "There are few dislocations or disturbances, and the strata are rarely inclined 
at angles greater than ten or fifteen degrees. Local uplifts are found among the mountains 
near Albuquerque and Santa Fé, where there are intrusions of trap, forming dykes ; but there 
> gae. on the al of the route near the 32d parallél, by W. P. Blake, 4to: Washington, 1855. 
2 See n the Geology of the route surveyed by Captain Gunnison, by Dr. Jas. Scheil. 
