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As a general rule the rocks of this group are harder and more crystalline in texture on the western than the eastern side of the Plains. In the former region they are also more calcareous in their composition.

Commencing towards the east we find the rocks of the Upper Carboniferous Group well exhibited in the western portions of Iowa, Missouri, and Arkansas, through various portions of the Cherokee, Choctaw, and Chickasaw Territories, and in Eastern and Southeastern Texas. Near the western line of Arkansas they exhibit a vertical development of about two thousand feet, forming here the Ozark Mountains. Thence they are traced almost continuously to Fort Gibson and Fort Washita, being interrupted in the last mentioned direction at two points only, one by a narrow strip of country occupied by strata of older date, and the other by the granitic protrusion already mentioned. At Fort Washita they disappear beneath the Cretaceous strata, but are exposed by denudation at several points between this point and the Upper Cross Timbers of Texas.

Directly west of the Cross Timbers the rocks of the Coal Measures are again met with, and thence extend uninterruptedly as far as the Clear Fork of the Brazos River, or about fifty miles west of Fort Belknap, where they appear in gently-rounded hills and ridges, and exhibit, by their exposed edges, a thickness of upwards of two thousand feet. A few miles west of this they disappear beneath the Cretaceous strata of the Plains, and are not again encountered in that direction until we approach the eastern base of the Rocky Mountains.

Farther south they are known to occur on both sides of the Rio Brazos for the distance of nearly a hundred miles; on the San Saba River, between Forts Mason and McKavett; and also near Fort McIntosh, on the Rio Grande.

Along the route traveled by your Expedition between Victoria and San Antonio heavy beds of coarse quartzose sandstone and conglomerate, agreeing lithologically in all respects with those of the Coal Measures farther north, were encountered. (Vide Journal) These rest unconformably beneath the strata of the Cretaceous System, and are known to contain in several localities beds of iron ore. They have been traced, as I am informed, many miles both north and south of the point where they were encountered by myself, and fifty miles south of San Antonio, on a stream known as the San Miguel River, contain workable seams of bituminous coal. Considering these, therefore, as also belonging to the upper division of the Carboniferous System, they would seem to indicate for the rocks of this group a continuous line of outcrop from Fort Belknap south as far as Fort McIntosh.