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proportion with sand or clay. In every detail of these and other generic characters they are different.

The words limestone and chalk are used in these pages as follows: Limestone is employed generically for species of widely different origin and structure. Specifically they may be of five kinds.

• (1) Breccias composed of more or less comminuted and cemented shells of ancient ocean bottoms or shore.
• (2) Concretions or segregations formed by the segregation of the lime in clays and sands after original deposition-rare in our rocks.
• (3) Chalky rocks or those composed of amorphous calcium carbonate, usually more or less foraminiferal, void of lamination, and of comparatively deep sea (not abyssal) origin. These may be hardened by metamorphism into firm limestones; hence the term chalky limestone will imply chalky origin, and the term chalk present chalky condition.
• (4) Laminated impure limestones, occurring as alternating beds in sands and clays, indicative of shallower origin than chalk.
• (5) Metamorphosed limestones, or any of the above which have undergone induration or other secondary change.

All laminated limestones thus far found are more or less arenaceous or argillaceous, further proving their origin to have been in shallower waters than those in which chalk is laid down.

THE LOWER OR COMANCHE SERIES

(AS SEEN IN THE COLORADO SECTION.)

In general the Comanche series is predominantly calcareous, although, as will be shown later, there are a few exceptions. Its rocks from bottom to top record a complete Ternary succession of strata, to-wit:

• 1. A lower stage of sandstones, shales, and other sedimentary deposits, representing prevalence of land with downward movement.
• 2. A middle stage, chiefly of limestone, representing prevalence of sea. and general quiescence and elaboration of calcareous organic formations.
• 3. An upper stage, and more of mechanical sediments, indicative of proximity to land.

The whole Comanche series is thus divided into three grand divisions, to-wit: The Trinity or Basal (sandy beds); the Fredericksburg or Medial (chiefly chalky beds), and the Washita or Upper division (impurer chalks and clays, alternating in stratification, becoming slightly arenaceous in the Denison region, but not so at Austin, for reasons explained later).

A.—THE TRINITY DIVISON.

This division is essentially arenaceous in composition, clastic in structure, and of littoral mechanical origin, being composed at its base of conglomerates or sands, the origin of every pebble of which can be located in the adjacent and more ancient strata of the Paleozoic region in the edges of Travis and Burnet counties, where the Trinity sands are in contact with the Paleozoic