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Lying unconformably beneath the rocks of the Cretaceous System, we have an assemblage of strata belonging to the Paleozoic Period, and composed, for the most part, of hard gray, blue, and white limestone; heavy bedded quartzose sandstone, conglomerate; millstone grit, and dark shales. Collectively they exhibit a thickness of between six and seven thousand feet, and are observed cropping out extensively both upon the eastern and western side of the Plains. On the east they are well exposed along the western borders of Iowa, Missouri, and Arkansas, and through various portions of the adjacent Indian Territories, and are observed extending southward through Texas as far as Fort McIntosh, on the Rio Grande. West of these they constitute the great floor or trough upon which are deposited the newer formations of the Plains, and after disappearing beneath the latter, are not again met with, save at a few isolated points, until we approach within one or two hundred miles of the western borders of the Atlantic slope, where they rise majestically to the height of several thousand feet in lofty mountain ranges.

Near the western line of Arkansas they exhibit a vertical development of near four thousand feet, and constitute a large portion of the Ozark Range. Thence, with the exception of a granitic protrusion about twenty miles in width and the area covered by the Cretaceous deposits of Fort Washita and the Cross Timbers already mentioned, they are traced continuously west and southwest to Fort Gibson, Fort Belknap, and the Clear Fork of the Brazos.

On the western side of the Plains they constitute the floors of several of the principal valleys. They also form the Guadalupe, Sacramento, Robledo, Fra Cristoval, and a large portion of the Organ and Horse Mountains. North of the district examined by myself, the Paleozoic rocks have been frequently encountered by others. Mr. Marcon mentions their occurrence in the mountains near Albuquerque. Lieut. Simpson met with them in latitude 36 degrees 12 minutes, longitude 108 degrees 52 minutes; Lieut. Abert, in latitude 37 degrees 15 minutes, longitude 104 degrees 35 minutes; while still farther north they have been recognized by Emory, Fremont, Parker, Stansburry, and other explorers.

The rocks of the Carboniferous System are separated by their lithological and paleontological characters into two distinct and well defined divisions:

Own's Report.