The region of country the geology of which forms the subject of the present report lays to the west and southwest of the State of Arkansas, and is comprised principally within the limits of the 29th and 35th degrees of latitude and the 94th and 108th degrees of longitude. It extends, on the one hand, from a line drawn from Indianola, on the Gulf of Mexico, by way of San Antonio de Bexar, Fort Inge, and Fort Clark, to Fort Davis, north to within a short distance of the Canadian River; and, on the other hand, from the western borders of the State of Arkansas and the district immediately south as far west as the Mimbres Mountains, situated sixty miles west of the Rio Grande.

Confined as my observations have been merely to detached portions of the region here indicated, and often compelled to conduct examinations hastily, I cannot hope to do more than present a general outline of its leading geological features. I shall, however, avail myself freely of notes taken during my former expeditions to the sources of the Brazos, Big and Little Wichita, and Red Rivers, and also refer to the observations of such other explorers as have had opportunities of visiting portions of the same field that I have not been able to examine myself.

The accompanying vertical section [Frontispiece] is intended to represent approximately the general character, thickness, and relative order of the different stratified formations encountered by myself in the district under consideration. It has been compiled from more than two hundred local sections taken in the course of my explorations in the Expedition under your charge and during former expeditions under Capt. R. B. Marcy. I trust it will he found as correct as could be expected from the necessarily rapid manner in which many of the examinations were conducted.

The first and second columns of the section contain the numbers and names of the several Geological Systems; the third, the subdivisions of the Systems into Formations; the fourth, their estimated thickness; the fifth, the colors employed to represent then; and the sixth, their general lithological character.