COAL FORMATIONS OF THE ROCKY MOUNTAINS.
We include under this head an extensive and rather indefinite region, extending from British America to Mexico.
This portion of our continent is a terra incognita, in a comparative sense, to the geologist; but the Paleozoic formations are known to exist around those towering peaks of granite to an indefinite extent, either concealed by the cretaceous and recent deposits, or in the obscurity of savage wilds. Many of our intrepid explorers of the West, however, have reported coal along the base of the Rocky Mountains, and numerous localities are pointed out, from the Black Hills in the North, across the Platte and Arkansas Rivers, to the Rio Grande, where true coal has been found.
Mr. Elisha Beadle, a miner of much experience from Pottsville, Penn sylvania, mentions the existence of true coal in the Black Hills, near Fort Laramie, in a letter published in 1853.
He says "the coal exists in regularly stratified sandstones, while the appearance of the formation is much the same as that in Schuylkill county and in the bituminous fields of the West." From a careful comparison of the various descriptions we have received of the coal formations lying along the eastern base of the Rocky Mountains, we are constrained to conclude them to be bituminous coals, but of an indefinite era. Whether they are a continuation of the great Appalachian formations or not, it is impossible at present to determine.
There appears to be an immense formation of brown coal, Tertiary coal, or lignite, lying between the known and developed portions of the true Carboniferous coal and the coal of the Rocky Mountains. Its range is immense, stretching from the Rio Grande to the head-waters of the Mis souri, possibly extending to the limits of the Palaeozoic formations in British America to the north, and extending along the eastern slopes of the Rocky Mountains, the Mexican Cordilleras, and the Andes of the South. It is, therefore, possible and probable that our true coal formation kdf the East continues its depreciation, as we have frequently noticed, until it terminates in mere lignites, and the true coal formations of the West may be independent basins of recent formation. The fact of these im
mense deposits of Tertiary coal or lignites existing in the western portions of the great basin, would indicate the absence of the necessary conditions required for the formation of true coal; and the thinning or depreciation of the Palaeozoic strata in that direction would justify such a conclusion, though ample evidence is offered of the existence of a shallow sea.
There is, however, a second theory which applies to the Western forma tions, but based on mere speculation in the absence of geological know ledge. This theory assumes that the true or carboniferous strata may underlie the Western fields of brown coal and lignites and the cretaceous strata of the prairies of the far West, as the true coals of Illinois underlie the Permian strata. This is doubtful, to say the least, though we intimated in a former chapter the possibility of the formation of the palaeozoic strata in a western as well as an eastern direction.
The true coal of the great • basin extends through Texas in a southern direction, and run to a point or comparatively narrow deposit in Mexico. It is found in Coahuila, New Leon, San Luis Potosi, and as far south as Vera Cruz and Oaxaca. It has been mined at Reveilla, on the left bank of the Salada River, about 125 miles above Camargo, by an American company. The coal is hard, bituminous, and stratified with sandstones. It has also been proved in Oaxaca and on the proposed route of the Tehuantepec Railroad.
It would appear from the foregoing facts that the ancient Appalachian Sea was not confined to the present North American Continent, but that its southwestern borders were along the eastern slopes of the mountains of Mexico and Yucatan.
We may, therefore, assume that the ancient sea was isolated,—that it had no connection originally with the Atlantic, but has been subsequently connected by the subsidence of the ancient coast-ranges to the south.
This interesting scientific question cannot be determined without more geological knowledge than we now possess of the western and southern limits of the great basin.