THE LOWER OR COMANCHE SERIES. 121 



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 ex- 

 ceeding 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 un- 

 derstand them to be, and often of oolitic structure. 



This alternation of softer marls and harder limestones produces the beau- 

 tiful 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 north- 

 west. 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 hori- 

 zon the name oolitic marls, f These marls have very little clay, and pack 

 when wet like f oiler'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 1*7 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. 



f The term oolitic is here used after Prestwich's definition, to-wit : "A compact light yel- 

 low 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 Ge- 

 ology, Chemical, Physical, and Stratigraphical, Vol. 1, p. 20.) 



