RE A TRES LO Nea, di A ai aL 
ROaATr Q COAT 65 
Eil LA Oe 
schistose, and yet very hard—harder than the red sandstone of the gypsum formation. The 
sandstone of the cliff at the Aztec range is red in some places, and again is grey, but it is very 
hard. Mr. Campbell considers it a light-colored sandstone, and it is possible that the light- 
colored grey sandstones which he observed south of San Francisco mountain are of the same 
age. In these, he remarks a resemblance to the light-colored or white and grey sandstones of 
the table-lands about Zuñi and of the Llano Estucado. 
Thickness of the strata.—The rocks we have been considering reach their greatest develop- 
ment along the eastern portion of the line, in the State of Arkansas, and within a few miles 
of its western borders. The greatest thickness stated is 2,000 feet—the height of Sugar- 
loaf mountain, where the strata are horizontal. This, however, is merely, as I under- 
stand, the height of the mountain above the general level of the country immediately around ; 
and, as the base of the strata or the carboniferous limestone is not exposed at that point, the 
true thickness of the formation is not yet ascertained. "The Sans Bois mountains are estimated 
to be from 2,000 to 2,500 feet high, and are probably composed entirely of carboniferous sand- 
stones and shales. Petit Jean mountain has a height of about 950 feet above the river. The 
outcrop of the coal-measures at the Sandia mountain must be, as we have seen above, about 
1,700 feet thick ; for, out of a combined thickness of 2,000 feet, the limestone occupies 300. 
Much further west, at the Aztec mountains, the whole series, including the limestone and an 
underlying bed of sandstone, is only about 600 feet thick. "The rocks have undoubtedly been 
greatly denuded ; yet it is certain that the thickness of the formation diminishes towards the 
west. 
Fossils.—Very few fossils were found in the sandstone strata, and most of them are referable 
to the lower division of the group, or to the period of the limestone. Ten miles up Little 
river, near Delaware mountain, Mr. Marcou cites the presence of fossils—Productus, Crinoids 
and Bellerophon, and bivalves—in the sandstone. Similar fossils were found in the creek at 
Camp No. 17. Very few fossil coal-plants were seen, and none were found or collected at the 
coal-beds. Between Camps 12 and 13 the road lay among hills of carboniferous sandstone, and 
five or six fragments of the stalks of Sigilaria were procured. A fine specimen is in the col- 
lection, and is the only fossil from the coal-measures, if we except specimens 100 and 101—a 
Lepidodendron—from Camp 47, Arroyo Truxillo. At Choctaw Agency, rolled fragments of 
carboniferous sandstone were found in a brook, containing Polypi and imprints of Productus ; 
also the imprints of Sigillaria and of Equisetum. 
Coal.—It appears that the outcrops of bituminous shales are quite abundant, and that coal is 
found at many places along the Arkansas, but generally in thin beds, only in a few instances 
thick enough to be economically worked. The usual coal-plants, with the exception of the 
specimens of Sigillaria, were not found; but this is not singular, as none of the beds were 
opened and explored. The most important of these localities of coal are enumerated in Chapter 
VIII, under the head of Coal. No coal was found in the Santa Fé mountains ; but the shales 
were very dark ; and Mr. Marcou ascertained that workable beds were found in the Manzanna 
mountains at the south, they being a continuation of the same chain. It is, however, by no 
means certain that this deposite is in the carboniferous formation ; it may be much more modern; 
and the probability is strengthened by the fact that, at about the same distance north of the 
line, the coal deposite of the Raton Pass is found, and was at first considered as the true coal, 
but afterwards was shown to be much more recent, impressions of leaves of dicotyledonous plants 
having been obtained there. 
It is possible that the coal found at Ojo Pescado, in the dark-colored bituminous shales, is 
true Carboniferous, but it is very doubtful. Neither coal nor bituminous shales appear to have 
been found west of this locality. Even the few observations which we have, force us to con- 
clude that the coal beds decrease in thickness towards the west ; and, indeed, that they probably 
totally disappear west of the Sandia mountains. 
9t 
