88 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
for example about Austin, the work has been done in great detail. An 
investigation of the source of the artesian water supply at San Antonio 
shows that the water is derived from the same series of beds that supply 
the wells at Waco, Fort Worth, and Dallas. There is, however, this 
important difference: At San Antonio the water is largely supplied by 
the Edwards limestone, heretofore supposed to be impervious and non- 
water bearing, while in the region of the above named cities the source 
of the water is pervious beds of sand. “It became apparent that this 
hitherto unappreciated water-bearing formation had great possibilities 
for supplying with either flowing or non-flowing 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 
towards the Pecos River, and even comprising the extensive summit 
region of the Edwards Plateau.” 
The geographical features of the region are (1) the Rio Grande Plain; 
(2) the Edwards Plateau; (3) the Balcones Scarp. “Broadly considered, 
they are a lowland plain inclining gently southeastward to the Gulf of 
Mexico, an upland plain rising gradually towards the northwest, and a 
rugged zone of separation which includes a quick ascent from plain to 
plain.” 
The Rio Grande Plain is characterized by a low relief, yet there are 
founds hills of considerable magnitude: buttes, old volcanic necks, and 
masses of basalt. “The Anacacho Plills, extending east and west in south- 
ern Kinney County, and constituting the most rugose part of the plain, 
are of still another type, consisting of a monoclinal plateau, or cuesta, 
sloping southward and presenting a steep scarp to the north.” 
Climatically, the plain is divided into an eastern or humid and sub- 
humid region, and a western or arid region. 
The Balcones Scarp, the position of which “is determined by a com- 
plex dislocation of the rocks, the Balcones fault,” is the dissected edge 
of the Edwards Plateau. 
The elevated Edwards Plateau merges into the Llano Estacada. There 
is no definite line of separation between them . The main drainage of the 
Edwards Plateau is to the east and southeast, and as the water-shed lies 
well to the westward, the erosion of the streams flowing into the Pecos 
is but moderate. The Plateau presents three simple topographic ele- 
ments, viz.: “the flat-topped summits,” “the breaks or slopes of its cren- 
ulated borders and canyoned valleys,” and “the stream ways.” The 
cap-rock is of the Edwards (caprina) limestone. 
As the streams of this region have an important bearing upon the 
underground flow of water, the Rio Erio has been selected as a type and 
described in detail. 
