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2a. THE BASAL OR ALTERNATING BEDS.The basal beds consist of thin arenaceous white limestones of a coarse, crystalline, and chalky aspect, sometimes slightly brecciated, but seldom exceeding one or two feet in thickness and of great uniformity in extent of stratification. These beds are separated by softer unconsolidated, magnesian, slightly argillaceous marls, resembling the yellow marls of France as I understand them to be, and often of oolitic structure.
This alternation of softer marls and harder limestones produces the beautiful bench and terrace topography of the western scarp of the Grand Prairie south of the Brazos River and east of the coal measures, which is especially well shown in Burnet County, southeast of Burnet, in western Travis, and numerous other places. They seem to be missing, however, in the northwest. While more or less very finely arenaceous and calcareous at the base, the quantity of sand in the mixed strata gradually diminishes upward, and the chalky lime increases until the culmination of the chalky bed recorded in the next division. The yellow magnesian strata also increase in thickness, and become very conspicuous in the middle portion of this lower subdivision often being from five to fifteen feet in thickness, as seen in the bluffs of Mount Bonnell north of the great fault. These magnesian limestones are soft enough to be cut with a knife, and are of a brownish yellow color. They alternate with similar strata of chalky limestones and yellow marls. The upper 100 feet of the basal subdivision of the Fredericksburg division, as seen at the top of Mount Bonnell, again present the unique stratification of the basal beds, the lime strata averaging about one foot in thickness.
The intervening yellow magnesian marls are soft and laminated, more or less siliceous, and composed of minute shells and concretions, which make it distinctly oolitic in character, and hence I propose for this stratigraphic horizon the name oolitic marls. These marls have very little clay, and pack when wet like fuller's earth. When properly understood they promise much, both from an economic and purely scientific standpoint.
They finally terminate in persistent beds of yellow marl abounding in a beautiful oyster, after which it is called the culminating horizon of Exogyra texana. From careful measurements of Mr. J. A. Taff, at Travis Peak, from"
Nos. 4 to 17 of the detailed section of Shovel Mountain, Burnet County, published by Dr. B. F. Shumard in the Transactions of the Academy of Science, St. Louis, Vol. 1, pp. 584 and 585, 1860, are typical of these alternating beds.
The term oolitic is here used after Prestwich's definition, to-wit: "A compact light yellow and gray carbonate of lime, often in the form of small rounded grains like the roe of a fish, at other times consisting of small comminuted fragments of shells." (Prestwich's Geology, Chemical, Physical, and Stratigraphical, Vol. 1, p. 20.)