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they may appear. They occur in the latter, however, in the Chickasaw Nation, as has been shown by the writer, where they consist of limestones. Certain limestones in the Burnet plexus, called Burnet marble by the writer, may ultimately prove to be of this age.
CARBONIFEROUS AND PERMO-TRIASSIC ROCKS.
EXTENT, SUBDIVISIONS, AND GENERAL RELATIONS.
The Carboniferous and Permo-Triassic rocks of the Central Province of Texas and Indian Territory are of great areal extent and importance. They are the principal strata adjacent to the interior borders of the Grand Prairie and are the buried floor upon which the Cretaceous rocks of the latter are laid down, as shown in many of the geologic sections in this paper.
The maximum thickness of the Carboniferous and Permo-Triassic rocks aggregates nearly 30,000 feet, and for convenience of discussion they may be for the present divided into three great provincial groups, as follows:
The upperrmost of these grand divisions, the Permo-Triassic, which is more fully discussed on a later page, consists of beds of red clays, sands, and impure limestone. The Permo-Carboniferous is largely made up of alternations of evenly bedded yellowish limestone, shales, sandstones, and conglomerates, and, as indicated by its name, is a transitional formation connecting the Carboniferous and the Permian. The Carboniferous proper is composed almost entirely of impure shales and sandstones with coal beds, with a thick bed of limestone near its base in the Indian Territory and Texas. These beds will not be discussed in detail, but some generalizations concerning their occurrence as a whole will be presented.
These formations are sediments wholly or in part derived from an old pre-Carboniferous Appalachian land, situated eastward of "
Am. Jour. Sci., 3d series, Vol. XLII, August, 1891, p. 121.
Am. Geologist, May, 1889, p. 3.
Since this chapter was written Mr. J. A. Taff, of the United States Geological Survey, has begun a thorough survey of the Indian Territory field, and his results will materially refine and complete the fragmentary knowledge we now possess of that interesting region.