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Comanche epoch, when the shore line was hundreds of miles to the west.

As this invasion was progressing the highland of the interior area was being degraded by erosion, and its débris, carried to the shore line, became the marginal sediments of the advancing sea. Along the eastern margin of the Central Province and beneath the western margin of the Grand Prairie, as more fully explained later in the section on structure, there was a range of mountainous protuberances, or rather the seaward cliff edge of a great plateau, which sloped steeply to the east. The old land to the west was a more level plateau beneath the Central and Plains provinces. The migration of the shore line across this narrower belt of greater slope occupied as great an interval o� time as that required for it to cross the entire plains region between the slope and the present east front of the Cordilleras.

The differences in the composition and stratigraphy of the beds of the three divisions of the Comanche series within the area of the Grand and Black prairies are due to the relations which the sediments of the old Cretaceous sea bore to this migrating shore line (see figs. 8, 11, 39, 40,43). The varying composition and near-shore character of the sediments of the Trinity division are due to the fact that they were laid down against the near-by steep slope of the subsiding land from which the material was derived. This slope was an eastern scarp of the old post-Carboniferous surface, elsewhere described as the Washita. paleoplain. These beds (of the Trinity division) are the only Cretaceous formations derived from a near-by shore line, or one which can be definitely located within the East-Central Province, and vary rapidly in composition and thickness proportionately to their proximity to or distance from it. On the other hand, most of the rocks of the Fredericksburg and succeeding divisions, within the Black and Grand prairies, were laid down within comparatively clear seas and at great distances from the land, long after the advancing littoral had conquered the barrier slope and while it was sweeping west across the wider regions of the Central and Great Plains provinces. Hence, relative to the deposits of the Trinity division, they are more uniform and calcareous in composition, less variable in thickness, and more widely distributed in areal extent.

This division includes the lower or initiatory beds of the Cretaceous formations of the Texas region. embracing all the rocks lying below the Walnut beds of the Fredericksburg division.

The rocks of this division are among the greatest of the water-bearing formations of Texas, and are of utmost importance in the