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Boston and several others of the loftier summits of the Ozark range of mountains, and there constitute a portion of the principal axis or line of greatest elevation between the Illinois and White Rivers. Immediately west and southwest of this they disappear beneath the arenaceous beds of the Coal Measures, but afterwards reappear at several points in the Cherokee and Choctaw Territories. In the latter Territory, about 100 miles southwest of Fort Smith, they are well exposed in a low ridge, which extends irregularly across the country for many miles in a southeasterly direction. Beyond this they are again seen to dip beneath the rocks of the Upper Carboniferous Group, and are no more met with in that direction during the remainder of the distance to Fort Belknap and the Clear Fork of the Brazos River.

On the San Saba River, at a point about midway between Fort Mason and Fort McKavett, the rocks of this group are seen cropping out extensively from beneath thick strata of the Cretaceous Period. At this locality the thickness of their exposed edges was estimated by myself at about two thousand five hundred feet. (Vide Journal.) The same outcrop is described by Roemer, who succeeded in tracing it many miles east of the point (on the San Antonio road) where it was encountered by myself.

Towards the west, strata belonging to this division of the Carboniferous System has been met with in the Organ and Fra Cristoval Mountains. In the Organ Mountains their exposed edges are exhibited in some places to the thickness of about three thousand feet. Commencing about twenty miles north of El Paso, they are traced in low hills and ridges along the western base of the igneous protrusions, of which these mountains are there mainly composed, to a point nearly due east of Fort Fillmore. Here they suddenly attain an elevation of about three thousand feet, but immediately afterwards descend, and thence northward appear at a few points only along the western base of the mountains until we arrive in the neighborhood of the San Augustine Pass, about fifteen miles east of Dona Ana, where they are exposed to the thickness of seven or eight hundred feet. A little north of this they again attain an elevation of several thousand feet, and from that point constitute, with the exception of the overlying Coal Measures already mentioned and an interrupted chain of igneous hills, the entire mass of the Organ Mountains as far north as the northern extremity of the Jornado del Muerto. Upon the western side of this plain they are again met with in the Fra Cristoval Mountain. In this mountain the thickness of their exposed edges was estimated approximately at about two thousand feet.

On both sides of the Plains north of the region examined by myself the rocks of this group have been abundantly encountered by other explorers. Towards the east they are mentioned by D. D. Owen as occurring along the western borders of Iowa and Missouri; while in the opposite direction they have been encountered by Mr. Marcon near Albuquerque, and by others at various points in the Rocky Mountains much farther north.