IN THE UPPER RIO GRANDE EMBAYilENT IN TEXAS. 59 



water in the river. Tlie other lies seventy feet above this and con- 

 sists of lenses of more regular form, and these also run into con- 

 tinuous layers. Seams and geodes of quartz which is not cherty, 

 but of crystalline structure, were also noted in these ledges. One 

 such seam was followed for nearly a mile. This quartz is associ- 

 ated with calcite, some of which has a dark amber color. This 

 has led to some desultory prospecting below Cedar creek i block 1). 



The siliceous segregations are characteristic of the Edwards 

 limestone almost everywhere in Texas, and the chert is often found 

 in greater abundance than is the case on Devil's river. It cannot 

 be regarded as indicating mineralization of anj' consequence. 



Frequently the ledges of the Devil's river limestone are bitu- 

 minous. When crushed they emit an oily odor. In the dry bed 

 of Pafford creek in survey 68, Block 1, about a quarter of a mile 

 west of the river, I found one such ledge which contains small 

 geodes filled with black and hard asphalt. These geodes have 

 originally been empty cavities in the rock, and crystals of calcite 

 have been formed on their walls before the filling of bitumen ac- 

 cumulated. This appears to have oozed in from the surrounding- 

 rock. Some of the geodes are still empty and only the calcite is 

 present. The asphalt is quite scarce. The geodes varj" in size 

 from one to two inches in diameter, and one seldom finds more 

 than two or three on a square rod of exposed surface. Nothing- 

 was seen suggesting bituminous accumulations in any valuable 

 quantity. 



Distribution. 



The Devil's river limestone is exposed along the entire distance 

 of the river from Camp Hudson down to the Rio Grande. North 

 of Deadman's creek the overlying beds have been cut away by 

 erosion on both sides of the river to beyond the limits of the New 

 York and Texas Land Company's surveys, but south of this, 

 owing to a low dip to the south, this limestone is less extensively 

 bared and forms the surface rock for only two or three miles away 

 from the river, or even less. In the region of Del Rio the forma- 

 tion disappears under higher beds, as we follow the river. The 

 south border of its area of outcrop here turns to the east. In this 

 direction it runs a sinuous course for fifty miles and more, follow- 

 ing in a general way the north limit of the New York and Texas 

 Land Company's surveys to the east of Turkey mountain. 



