100 A GEOLOGICAI. SUEVEY OF LANDS 



In this connection attention should be called to the fact that 

 the strata which contain the gas in the Mula creek well are sedi- 

 ments of the same or of very nearly the same age as the strata 

 which yield oil in the Boulder district in Colorado and at Corsi- 

 cana in Texas. In both of these places the oil is associated with 

 gas, though this has been only occasionally found at Corsicana. In 

 the Boulder district gas is of somewhat more common occurrence 

 than at Corsicana. In the Boulder field the formations are more 

 strongly folded than in the country southeast of Eagle Pass, but 

 at Corsicana the .structure is about the same. Everything con- 

 sidered, the prospect of finding oil on that part of the Lampasitas 

 arch covered by the Escondido beds can not be regarded as alto- 

 getherunpromising. Prospect holes should go down to the Upson 

 clay. Below the top of this clay there is no rock sufficiently porous 

 to warrant exploration. 



SALTPETER. 



The saltpeter which has been found in a cave on Devil's river 

 is a rather exceptional mineral occurrence. It has been formed in 

 its present place by a natural process from human excrements 

 and food waste, originally left in the accumulating earth and rub- 

 bish on the floor of an aboriginal dwelling place. Saltpeter of 

 commerce is in part manufactured by mixing refuse animal sub- 

 stances with lime and earth and leaving the mass to decay. The 

 lime in these caves was produced in connection with the heating 

 of limestone rocks in the preparation of food. Great heaps of 

 these partly calcined stones are seen on the ground. The accum- 

 ulated rubbish which is found on the floor of the old dwelling 

 place averages about five feet in thickness and covers an area 136 

 feet long and about 34 feet wide, or 2,890 square feet. From sam- 

 ples taken at respectively one, two, three, four, five and six feet 

 below the surface, it was ascertained that the saltpeter is present 

 only in the upper three feet. Into this stratum it has evidently 

 been brought by capillary rise of moisture, drawn from a lime- 

 stone shelf below and dissipated by evaporation from the surface 

 of the debris. Eough analyses of each of the samples taken are as 

 follows: 



Percent of Saltpeter. 



Uppermost foot 20 



Second foot below surface 7 



