THE LOWER OR COMANCHE SERIES. 119 



beds of tiie Comanche series. This is composed of a solidified mass of large 

 grypheate oyster shells resembling the dilate species figured by Marcou as 

 Gryphcea dilatata, but not yet positively determined. These were found to 

 form a stratum seven or eight feet in thickness, just below the junction of 

 Post Oak and Cow Creeks. Accompanying the Gryphaea breccia there is 

 also the first appearance of another conspicuous feature in the Comanche 

 series, i. e., an excess of epsom salts, or magnesian sulphate. The oyster 

 shells are being rapidly cemented into massive limestone beds, or decomposing 

 into a powdered earthy substance accompanied by incrustations of epsom 

 salts (epsomite). This magnesian feature, which becomes more conspicuous 

 higher in the series, is a fine illustration of an instance of the conversion of 

 a shell limestone into dolomite by an alteration subsequent to the formation 

 of the original rock, as has been recorded by Irish geologists.* 



In places throughout the sands are occasional patches of red and greenish- 

 white clays, resembling very much the characteristic features of the red beds 

 of the lower formations, sometimes accompanied by lignite and fossil bones. 

 The cause of these discolorations has not been studied. There are from 200 

 to 300 feet of these arenaceous Trinity beds in the Colorado section, at the 

 top of which appears a fossiliferous horizon — the first or lowest appearance 

 of Jfonopleura (Caprotinci) and Requienia — which we assume to be the 

 beginning of the second division of the Comanche series. Thus the Trinity 

 beds in the Colorado section are seen to be composed of locally derived debris, 

 which, as the waters deepened, became more and more comminuted and 

 calcareous, until the sand grains at the top are almost imperceptible to the 

 eye, and the whole mass becomes quite chalky and magnesian in appearance. 

 As shown elsewhere, these basal arenaceous beds everywhere vary with the 

 shore line upon which they were laid down, and are different in composition 

 and detail in the Rrazos and Arkansas sections. 



Economic Use of the Trinity Sand. — Notwithstanding its sterility of 

 soils, it is no exaggeration to say that they constitute one of the most valuable 

 rock sheets in the State, inasmuch as their porous beds, dipping between two 

 impervious strata to the eastward, constitute a ready medium for the perco- 

 lation and transportation of most of the water that falls upon them to greater 

 depths, where from a lower surface altitude than their outcrop they are pene- 

 trated by artesian wells drilled from above, and bountifully supply the artesian 

 water now being secured all along the country adjacent to the margins of 

 the Black and Grand prairies at Fort Worth, Taylor, Waco, Austin, and 

 other places from Red River to the Rio Grande. Thus it is that a stratum 

 which outcrops in one region as a sterile sandy district, in reality proves a 



*See Prestwich's "Geology, Chemical, Physical and Stratigraphical," Vol. 1, pp. 113, 114, 



