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through which appear many bright colored flowers. This is the southern end of the flora of the Great Plains region, which continues far northward.

Although there is much agriculture in the wide, fertile, plaza canyon valleys indenting the plateau, especially in Blanco, Gillespie, Comal, and Kendall counties, the slopes and summits constituting the larger part of the area are not adapted to agriculture, owing to the rocky character of the soil, the semiaridity of the country, and the impossibility of irrigation. So little are they fitted for agriculture that the extent of the summit of the Edwards Plateau can almost be traced upon the map by the scarcity of post offices and other evidences of population. They constitute, however, good grazing country and support many large sheep and cattle ranches.

The rocks of the earth forma system of natural works by which water is collected, stored, and distributed. They constitute the basins, reservoirs, conduits, and other portions of the plant for retaining and distributing underground water.

The details of natural waterworks differ in different places in the same way that the details of artificial systems differ in different plants, but the distribution in both is governed by the common principles of hydrostatics. The efficiency of a natural system is determined by the texture of the rocks and the geologic structure of the region, so that an understanding of the availability of underground water in any region necessitates a knowledge of the elementary geology of that region.

It is neither convenient nor advisable here to discuss minutely the source, storage, and distribution of underground water, but in order that the subject may be understood, we shall. give a brief explanation of the elementary principles governing its occurrence.

• (1) The primary source of all underground water is the rainfall.
• (2) Rocks imbibe water. Imbibition may take place by percolation and by absorption. By percolation is meant the process by which water proceeds through cavities, cracks, fissures, or other breaks in the continuity of the underlying rocks. Absorption takes place when water enters the small interstices of the rock.
• (3) Water can flow in rocks, and the rapidity of this flow is known as the capacity for transmission.
• (4) Different kinds of rock have different capacities for imbibing and transmitting water. These capacities are not proportional to one another, but vary independently, according to the kind of rock.
• (5) All water entering the earth tends to gravitate downward along lines of least resistance (easiest transmission).
• (6) The only ordinary agency by which rock sheets may be naturally drained of their water is gravity. Gravity drainage may be of two kinds, direct or artesian.
  • (a) Direct drainage is that by which water