GYPSUM FORMATION—-DISTURBANCES OF THE STRATA. 73 
mite is noted as three or four feet high and cavernous, with rose-colored beds, as at Salins. 
(See notes, September 11.) The red clays occur above and below this dolomite. In ascending 
the bluffs of the Llano Estacado, between Camps 41 and 42, strata of dolomite from three to 
five feet thick were found, and at Camp 42 it was very abundant. A sandy dolomite is noted 
as present in the face of the bluffs near Cuesta; and I find a large specimen of compact dolo- 
mite in the collection (No. 61) taken from San Antonio, southwest of Cuesta, but which does 
not appear to be mentioned in the descriptions. This rock is bituminous, and may pertain to 
the Lower Carboniferous. The dolomites do not appear to have been found in the valley of the 
Rio Grande, nor in any quantity west of the Sierra Madre, until the base of the volcano of 
San Francisco was reached. We may, perhaps, except the beds of yellowish limestone asso- 
. Ciated with the bituminous shales at Ojo Pescado, but which are very probably of carboniferous 
age. The strata of magnesian limestone beyond the Colorado Chiquito are mentioned in the 
general description of the carboniferous formation, and their probable carboniferous age is 
noted. According to Mr. Marcou, they occur in concordant stratification with the strata of the 
“¿New Red ” sandstone with which they alternate, and attain a thickness of six inches to one foot. 
This description must refer to other strata than those seen along Cañon Diablo, which were very 
much thicker, forming the walls of the ravine, one hundred feet deep, but which were con- 
formable with the red strata. It is very probable that the upper beds are interstratified with 
sandstone. A specimen in the collection from the Colorado Chiquito, and probably taken from 
this locality, much resembles the specimen from San Antonio. The collection contains many 
specimens of the magnesian limestones, and they are all described in Chapter X. (See Nos. 
60, 61, 62, 63, and 64.) With the exception of the specimen from near Camp 30, they are all 
hard and compact, and of light grey or buff color. 
Thickness of the formation.—In a formation of this character—one evidently resulting from 
the metamorphosis or change of strata, entirely different in color and mineral ingredients, and 
which we cannot separate exactly from formations either above or below—it is of course not 
possible to state definitely the thickness, or to give an average thickness for the whole series of 
beds as a distinct formation. The thickness of the red strata appears to be the greatest on the 
eastern slope of the Albuquerque mountains, under the Llano Estacado, and perhaps near the 
point where the gypsum is most abundant. There is, however, no valley of erosion in these 
horizontal strata deep enough to expose all the beds, nor is there at any point an uplift which 
brings them all into view; the thickness, therefore, must remain unknown. The same obser- 
vation will apply to the strata west of the Sierra Madre and in the valley of the Rio Grande ; 
there are no erosions of a depth sufficient to develop the whole thickness of the formation down 
to strata of undoubted Carboniferous age. The exposures at any point appear to have never 
Shown a greater thickness than two or three hundred feet, the thickness of the white sandstones 
and chalky limestones of the Llano being subtracted from the depths of the valleys. Mr. 
Marcou, however, in his notes, states that the Trias has a thickness of four or five thousand 
feet, (October 9 ;) and again, that the thickness of the variegated marls, or the superior part 
of the American Trias, is considerable—not less than three hundred feet above the gypsum. 
In the Resumé, he states that the lower division of the *' Trias” attains a thickness of two to 
three thousand feet, the middle of about fifteen hundred, and the upper of fifteen hundred ; 
making for the whole a thickness of five to six thousand feet.! It is probable that these are 
estimates based upon the apparent thickness of the beds as they were passed successively in 
ascending the valley of the Washita and Canadian, and in descending from the Sierra Madre 
west along the valley of the Colorado Chiquito. | ; 
It is certainly very important to determine as nearly as possible the thickness of these strata ; 
but this does not appear possible, as neither the upper nor lower limits can yet be determined. 
Disturbances of the strata.—The strata were generally found to be horizontal, or nearly so, 
being conformable with the cretaceous strata of the Llano Estacado. The first observation on 
1 Resumé, &c. Report of Lieutenant A. W. Whipple, 8vo., House Doc. 129, pages 42 and 43. 
10 £ ; 
