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The strata of the Lower Cretaceous, or Comanche series, form the surface and underlying rocks of the Grand Prairie and Western Cross Timbers. The various beds belong to three divisions, as follows, beginning with the lowest: Trinity, Fredericksburg, and Washita. The general characteristics of these divisions are as follows:

The Trinity division is especially marked by strata of friable white packsands, which do not occur in the other divisions and which in places constitute nearly the entire rocks of the division. In some places, especially south of the Brazos, these sands alternate with manly clays and chalky and clastic limestones, the latter being composed of minute shells or fragmental particles of shells and sands having a lithologic and paleontologic individuality by which they can usually be readily distinguished. All the calcareous strata are white or yellowish and occur in numerous persistent alternations of bard and soft strata of various thicknesses.

The rocks of the Fredericksburg division in the typical area of occurrence are almost entirely chalky limestone, initiated by beds of manly clay, which grade into the limestone.

The rocks of the Washita division include the beds between the top of the Edwards limestone of the Fredericksburg division and the coarse sands of the Woodbine (Dakota).

The sediments of the Washita division, while generally light in color in their lower half, show darker tones and greater ferrugination of rocks toward the top. They are composed largely of alternations of marly clays and firmer layers of limestone. The limestones of this division, while slightly resembling others in the series, have a sufficient. proportion of grit, and sometimes of iron, to make them relatively impure. The beds are successively shallower in origin in ascending series.

In general, rocks of the Trinity division were laid down upon a subsiding bottom of a former land surface; rocks of the Fredericksburg division (in this portion of Texas) upon a stationary offshore bottom of Trinity sediments; and rocks of the Washita division upon a shallowing bottom of Fredericksburg sediments.

All rocks of the Comanche series are sea sediments-deposits laid down in the ocean while it was invading a land area extending from east of the present Black Prairie north to the Ouachita uplifts and west to the Rocky Mountains, across the East-Central, Central, and Plains provinces of Texas. This invasion, first recognizable at the very beginning of the Comanche epoch, when the shore line was at or east of the eastern margin of the Black Prairie, ended at the close of the