72 GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
fully represented, as indeed we know they are not, when we compare the descriptions with the 
collection. 
We have thus seen that strata of various colors are found intercalated, and that the red marls 
and sandstones predominate. It also appears that the red color is most common or striking 
along the valley of the Canadian before the bluffs of the Llano are reached, and again on the 
western limits of the formation along the Colorado Chiquito. 
Gypsum.—In the preceding description of the sandstones and marls of the formation, an 
account of the various beds of gypsum was purposely avoided, in order that they might be 
more directly compared. Many specimens of this mineral are in the collection, and are par- 
ticularly described in Chapter X ; and further details respecting the quantity of the mineral, 
and its uses in the arts, will be found in Chapter VIII. The part of the route near which 
it appears to occur in the greatest abundance is along the valley of the False Washita, about 
northeast from the Witchita mountains. Most of the specimens are from Gypsum and Comet 
creeks, and the vicinity of Camp 30. 
At this point a bed twenty-five feet thick was found, and it has already been mentioned, and 
its association with the red sandstone and shales given. Between Camps 33 and 34, beds from 
one to twenty feet thick were found above strata of magnesian limestone, the bed at Camp 30 
being below. These beds of gypsum are described as white, and traversed with rose-colored 
and grey veins; and the bed at Camp 30 is amorphous and white. Near Camp 41, beds from 
six inches to one foot in thickness are found ; and beyond this point to the valley of the Rio 
Grande, very little gypsum appears to have been seen. One specimen in the collection is from 
San Antonio, the village at the eastern base of the Sandia mountains. There are no specimens 
in the collection from any point west of this locality, nor are there many observations on the 
gypsum. It does not appear to have been abundant beyond the valley of the Rio Grande, 
where we know that it occurs, and is used in considerable quantities as a substitute for glass. 
Its presence in the red sandstone and marl near the summit of the pass, in the Sierra Madre, is 
mentioned, but the thickness or extent of the bed, if indeed it forms one, is not given. (See 
notes, November 18.) It occurs in nodules between Camps 71 and 72, and near Camp 75 in 
crystals, distributed in the red marl and sandstone. It was also seen at the crossing of the 
Colorado Chiquito in thick kidney-shaped masses in the red sandstone. 
Although an enormous amount of gypsum is exposed along the route, the deposites do not 
equal in their extent and quantity those found by Captain Pope along the Pecos river and 
Delaware creek, on the route near the 32d parallel. There, according to Captain Pope, the 
beds are fifty feet thick, and form bluffs, which are full of caverns.! 
We must be careful to avoid the impression that the gypsum beds are continuous throughout 
the formation, or that they retain their thickness for great distances. The facts show that the 
thick beds are confined to a small portion of the route. Although we find the red sandstones 
on both sides of the mountains bordering the valley of the Rio Grande, and the strata appear 
to be geologically equivalent, there is a great difference between the different portions in respect 
to the amount of gypsum, and the form in which it occurs. It predominates on the Atlantic 
slope of the mountains, and is most abundant a few miles east of the last outliers of the table- 
land of the Llano Estacado, in the vicinity of Gypsum and Comet creeks , and near the cretaceous 
deposites of Camp 31. The term Gypsum formation thus applies more appropriately to the 
eastern than to the western portions of this wide-spread deposite of red marls, clays, and sand- 
stones. 
Dolomite. —Magnesian limestone or dolomite appears to form an important part of the 
formation, and is closely associated with the gypsum. The bed of dolomite which overlies 
the gypsum at Camp 30 is argillaceous, cavernous, and of a reddish color. A similar rock 
was again observed between Camps 32 and 33. Between Camps 40 and 41 a bank of dolo- 
' Report of Captain Pope, United States Topographical Engineers; also, report on the Geology of the route: U. 8. 
Pacific Railroad Exploration and Surveys. Washington: 1856. 
