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nature, such as the Government has deemed expedient to collect in other artesian regions. Such facts as are herein presented have been collected incidentally by the writers while making private reconnaissances or official geologic surveys in the region, and through correspondence with citizens. Accordingly, this report is not to be considered as a statistical or engineering paper, but rather one which deals with the geologic side of the underground water question.

The source of the San Antonio water supply has been ascertained by detailed studies of the structure and outcrop of the water bearing beds in the adjacent regions. A detailed map of the Austin quadrangle was made, and the thickness, sequence, paleontology, mineral composition, and water capacity of every portion of the Cretaceous section along the Colorado River were ascertained. A reconnaissance was next made southwestward from Austin via Fredericksburg, Kerrville, and the headwaters of the Frio and Nueces to Fort Clark, Texas, for the purpose of studying minutely the variation of the rock sheets outcropping in that direction, which, owing to the direction of their dip, must necessarily underlie the city of San Antonio. At the southeastern end of this line detailed studies and mapping were resumed upon the Nueces, Brackett, and Uvalde quadrangles. By this method a base line of geologic sections, so to speak, was established along the strike of the outcrop of the Lower Cretaceous formations, the sections at the extremities of which were determined with the greatest accuracy possible, while check sections have been made at numerous intervening points along the line.

The result of this work was the discovery that, while these well waters come from the same series of beds that supply the artesian wells of the Waco, Fort Worth, and Dallas regions north of the Colorado, their occurrence presents some important differences of detail. Instead of having their immediate source in beds of porous sands, like the wells about Waco, they are derived largely from the Edwards limestone, hitherto supposed to be one of the most impervious formations of the whole Cretaceous section.

It became apparent that this hitherto unappreciated water-bearing formation had great possibilities for supplying with either flowing or nonflowing wells a large area of country lying between Austin and San Antonio, extending west of the San Antonio River along the northern margin of the Rio Grande Plain toward the Pecos Piver, and even comprising the extensive summit region of the Edwards Plateau.

There are also many remarkable springs in that portion of the Rio Grande Plains and Edwards Plateau lying between the Colorado and Pecos rivers, which will be discussed in this paper. These belong to two distinct classes, each characterizing one of the geographic subdivisions mentioned, aid each exhibiting a method of escape of the underground water of the region. One line of these springs follows approximately the margin of the Rio Grande Plain, close to the line of