90

surrounding it. (See Pls. XIV and LXVI.) They consist of ferruginous brown sandstones alternating with limestone, as shown in the following section, made in Llano County by Walcott.

These rocks were originally reconnoitered by Roemer, and were further studied by Shumard, but Walcott presented the first classification, showing their unconformity upon the underlying Algonkian, giving measurements of their thickness, and demonstrating that they represent only the Middle and Upper Cambrian stages. Comstock has later given much detail of the distribution of these rocks, but has erroneously assigned their basal portion to the Lower Cambrian.

Rocks of Cambrian age may also occur in the Wichita Mountains (Bain) and in the Arbuckle Hills of the Ouachita system, as exposed in Washita Canyon between Dougherty and Berwin, Indian Territory. The details of this latter section have not been minutely studied, but from fossils collected by the writer, Prof. H. S. Williams determined some of probable Trenton age.

Ordovician or Lower Silurian rocks, composed of massive limestones and dolomite accompanied by cherts, occur in the Burnet, Wichita, Tishomingo, and Ozark regions. These have been studied in the Burnet district by Roemer, Shumard, Walcott, Comstock, and the writer. These limestones occur principally around the peripheral portion of the Burnet basin, in Burnet, Blanco, Gillespie, Mason, Kimball, McCulloch, and San Saba counties. Their contacts with the overlying Carboniferous and underlying Cambrian are well shown. The only estimate of thickness is the section measured by Walcott, who gives 1,145 feet.

Ordovician rocks also occur, as has been shown by the writer and by Vaughan, in the east-west folds in the Arbuckle and Wichita ranges of the Ouachita system, situated west of the Missouri, Kansas and Texas Railway, where they supplant as the chief mountain-making material the Carboniferous, which predominates to the eastward.

True Silurian (Upper Silurian) rocks have not been recognized in the substructure of the region south of the Ouachita Mountains, although "

Am. Jour. Sci., 3d ser., Vol. XXVIII, December, 1884, p. 432.

Texas, etc., Bonn, 1849; Die Kreidebildungen von Texas, etc., Bonn, 1852.

Trans. Acad. Sci., St. Louis, Vol. I, 1860, pp. 672, 673; Am. Jour. Sci., 3d ser., Vol. XXXII, 1861, p. 213.

Am. Jour. Sci., 2d ser., Vol. XXVIII, December, 1884, pp. 432-433.

First and Second Reports of the Texas Geological Survey, Austin, Texas, 1890 and 1891.

Am. Jour. Sci., 3d series, Vol. X LII, August, 1891, p.121.

Id., Vol. XXVIII. December, 1884, p. 433.

Id., Vol. XLII, August, 1891, p.121.

MSS.