484 



SCIENCE. 



[N. S. Vol. IX. No. 222. 



agency of rains. By the subsequent evapora- 

 tion of the water a superflcial cement or crust 

 is formed which may involve pebbles distributed 

 over the surface — this is 'tepetete.' 



The igneous rocks of the Rio Grande Plain 

 occur ' along the interior margin.' They are 

 basic. The rock from Pilot Knob has been de- 

 scribed as nephaline-basalt.* 



The arrangement of the strata is next con- 

 sidered. The Oetaceous rocks are shown to 

 have great persistency, while their dip towards 

 the coast is but slight. The Balcones fault zone 

 has already been mentioned as forming the 

 ' abrupt southern termination of the Edwards 

 Plateau.' " The strata on the seaward side of 

 the faults have been dropped down, so that any 

 particular stratum — the top of the Edwards lime- 

 stone, for instance — lies 500 to 1,000 feet lower 

 on the coastward or downthrow side of the 

 fracture than on the interior or upthrow side." 

 It should, however, be borne in mind " that the 

 fault zone really consists of many faults, having 

 subparallel directions all concentrated in a nar- 

 row belt of country." The displacement at 

 Mt. Bonnell, on the Colorado above Austin, is 

 such that the Eagle Ford shales of the Gulf se- 

 ries are brought in contact with the Glen Rose 

 beds of the Comanche series. 



Our authors now enter upon a discussion of 

 the water capacity of the various rock sheets. 

 The impervious layers are, of course, non- 

 water-bearing, and to this class, south of the 

 Colorado River, belong nearly all the Creta- 

 ceous rocks above the Edwards limestone. On 

 the other hand, "rocks of open texture, such 

 as sands, conglomerates, porous and chalky 

 limestones, and massive rocks broken by joints, 

 fissures, honeycombs or other openings, are 

 usually water-bearing. These are mostly found 

 below the Del Rio clay." The proof of the 

 water-bearing property of the Edwards lime- 

 stones is supplied by the "great springs * * * 

 bursting out of them at the head waters of the 

 Llano, Guadalupe, Frio and Neuces Rivers ;" by 

 the artesian well records at Manor, San Marcos 

 and San Antonio, and by the ordinary wells of 

 the Edwards Plateau. The distribution of water 



* J. F. Kemp. See Pilot Knob : ' A Marine Cre- 

 taceous Volcano, ' by Robt. J. Hill and J. F. Kemp. 

 Amer. Geologist, Vol. VI., 1890, p. 292. 



is facilitated also by the honeycomb and cavern- 

 ous character of certain limestone layers. The 

 opinion is expressed that the Travis Peak and 

 Gillespie formations, at the very base, contain 

 "a greater quantity of water than any other 

 beds of the Comanche series." 



The underground water of the region is next 

 taken up : (1) The waters of the Edwards 

 Plateau ; (2) The waters of the Rio Grande 

 Plain. In many instances non- flowing wells, 

 springs and artesian wells receive detailed 

 treatment ; of the latter logs are frequently 

 given, as in the case of the Austin wells, the 

 San Marcos well, the Manor well and those of 

 San Antonio. Under the chemical qualities of 

 the waters mention should be made of the ex- 

 cellent analyses of waters from Austin and vi- 

 cinity, made by Dr. Henry Winston Harper, of 

 the University of Texas. 



Of the fissure springs, those at San Marcos, 

 San Antonio, New Braunfels, Austin, etc., are 

 well known. On p. 311 is given a table of the 

 discharge of the various spring rivers, the San 

 Marcos reaching 57,522,200 gallons in twenty- 

 four hours, and the Comal 221,981,932 gallons 

 in the same time. From a study of the strata 

 and their faulting the authors conclude that 

 ' ' these waters come from the deep-seated rocks 

 and are forced to the surface by hydrostatic 

 pressure. Hence they are artesian in nature 

 and constitute natural artesian wells." Taking 

 into consideration the color, taste, temperature, 

 volume, freedom from sulphuretted hydrogen, 

 the conclusion is reached that "their water is 

 derived from either the ' sweet water ' horizon 

 of the Edwards formation or the Travis Peak 

 sands, that is, they have the same source as the 

 purer waters of the artesian wells." 



As to the source of the underground waters, 

 the fact that the Pecos breaks the continuity of 

 the strata renders it impossible, as the authors 

 point out, to consider the Rocky Mountain 

 region in this connection. They contend, on the 

 other hand, that the Plateau of the Plains is the 

 real source; that "much of the rain water is 

 caught directly upon the edges of the Glen Rose 

 and lower beds which outcrop along the west- 

 ern and northern summits, breaks and margins 

 of the Plateau * * * at an elevation higher 

 than that of their embedded continuation alona: 



