Comanche-summit (Johnson County), 650 feet above the level of the Rio Brazos, where they appear under the form of white hippurite limestones, overlying a bed of grey limestones, whose strata extend about 70 yards westward and form a low slope; they are filled to the depth of 30 feet with Exogyra Texana, Holaster simplex, Lima Wacoensis, and other organic remains. They alternate frequently with accumulations of Gryphoea Pitcheri and Exogyra Texana, and the total thickness of this group is 75 feet. On the south side of the hill, the Gryphaea-beds are 200 feet thick. The upper portions of the hill include a small quantity of Exogyra Texana, Janira occidentalis, Lima Wacoensis, Holaster simplex, and a Toxaster. The limestones above these strata abound in Hippurites, Caprotinoe, &c. The Texan Tertiaries consist of sandstones and limestones, the former frequently impregnated with a large proportion of oxide of iron. Remains of Zeuglodon cetoides have been found in the hill-rangeof the Rio Colorado, and bones of Mastodon giganteus and Elephas primigenius and other large quadrupeds on the banks of the Rio Brazos.
The calcareous sandstones and dolomitic limestones of the Potsdam period are of frequent occurrence in the western region of the State. They are rich in silica, and furnish marbles of superior quality.
The Azoic rocks include extensive beds of magnetic iron-ore. Enormous loose masses of this ore are spread over the surface about Johnson's Creek (Llano County); a hill, rising 45 feet above the level of the River Llano, is one solid mass of iron-ore; its extension in depth is still unknown. The ores, very analogous to those of the Iron Mountain of Missouri, are partly magnetic, partly specular oxide of iron; they contain on an average 96.890 per cent. of per- and protoxide of iron, with 2.818 per cent. of insoluble siliceous substances, and give 74.93 per cent. of metallic iron. This iron region is surrounded on all sides by ridges of granite, intersected by veins of quartz and associated with red felspar, gneiss, talc, and chloritic slates. Wood abounds in the environs, and limestone and steatite may be easily procured in abundance, the extreme indolence of the inhabitants, who find an easy and profitable employment in breeding cattle, sheep, and horses, being the only obstacle to the establishment of extensive iron-works.
Superficial indications of the existence of great quantities of petroleum below the surface are not unfrequent in the environs of the "Four Lakes" (Harding County). The acidulated waters of the lakes, and those of the springs surrounding them, are impregnated with carburetted hydrogen gas and petroleum. This locality belongs to the Miocene period, forming a zone 50-75 miles in breadth and extending several hundred miles in length along the coast. Mr. Roessler has brought from the sources of the Rio Brazos a block of meteoric iron weighing 315 lbs.
