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STRATIGRAPHY OF THE COMANCHE SERIES IN GENERAL.

From the foregoing facts it is evident that the Comanche series possesses a well defined lithologic and stratigraphic history. Its lower division is essentially sandy, but becomes less and less so and more calcareous as the bottom upon which it was laid down subsided.

The lithology of the Comanche series is predominantly calcareous and is marked by several essentially chalky horizons. There are also magnesian and arenaceous beds, but these are modified in color and appearance by the predominance of the accessory chalky matter. In color the tint is chalk white, yellow, cream-colored, and occasionally the white rock weathers into a dark grey, and not even in a single case are these rocks concretionary as recently recorded, unless it is in a few feet of the Denison beds above mentioned.

Portions of the section are stratified into bands of one foot or more, but a large majority of the strata are massive, while the whole series, except a few alternating marls and layers of the Trinity, are remarkably free from lamination.

The alternating beds of the Basal subdivision of the Fredericksburg clearly show a deeper sea condition of origin than the Trinity, but not as deep as the chalk of the Comanche Peak and Caprina limestone subdivisions, which were deposited in very deep and quiet waters. After the latter there is a hiatus in our knowledge, but the Washita division reveals an elevation of the ocean's bottom as slow and positive as is the subsidence recorded in the other basal divisions. In brief, there is recorded

• (1) a long continued subsidence, during which nearly one thousand feet of deepening uniform sediments were laid down over vast areas;
• (2) a long continued deep sea condition, in which four or five hundred feet or more of chalks were deposited;
• (3) an elevation in which from three hundred to five hundred feet of shallowing sediments were deposited (the Washita division).

There can be but little doubt that the rocks now composing the Comanche series were elevated into dry land, that the succeeding land epoch continued probably as long as the time of deposition of either of the including series, and that the rocks of the Upper series were largely derived from the underlying Comanche strata, and laid down during an entirely different and later oceanic subsidence.

It is also evident that the Comanche series thickens to the southwestward and thins northward. Mr. Taff's measurements along the Colorado section make it 1500 feet, but both the top and basal beds are locally curtailed there.

Throughout its extent the rocks are jointed, and along their eastern margin greatly broken and faulted parallel to their strike, while occasional areas of igneous rocks protrude through the series southwest of Austin.