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the railroad from Austin to Del Rio. The other class embraces the springs of the canyons of the Edwards Plateau.

In order to understand the occurrences of water of the San Antonio system, it is necessary to carry in mind

• (1) The geography of the region.
• (2) The simple laws of the occurrence and distribution of underground water.
• (3) The composition, variation, and arrangement of the rocks underlying the region and affecting the distribution of water.

The general map (Pl. XXI) will illustrate the geographic relations of

the Edwards Plateau to the adjacent and lower Rio Grande Plain. The local sections and descriptions of the formations given in the geologic portion of this paper (pp. 215-260) can be referred to for all necessary geologic data, such as structure and variation in thickness.

GEOGRAPHY OF THE REGION.

The traveler by rail from Austin via San Antonio to Del Rio, on the Rio Grande, sees from the car window two conspicuous regions, each having its peculiar geographic features. On the left is a gently undulating plain whose margin the railroad follows. This is here termed