82 A GEOLOGICAL SUBVEY OF LANDS 



mountains in the Mula creek basin, it is certainly not less than 

 500 feet. 



The bituminous sandstones noted on the Nueces river in this 

 formation no doubt underlie some of the lands in the northwest 

 part of Zavalla county, but the presence of the bitumen cannot be 

 ascertained except by exploration. Wells made on these lands 

 merit attention for the detection of this product at reasonable 

 depths. The synclines, or downward folds, in which the asphalt 

 is most apt to have accumulated, are concealed, if there be any, 

 by the overlying Tertiary deposits. 



THE TERTIARY SEDIMENTS. 



The deposits belonging to this age cover the older sediments 

 on most of the lands in Zavalla and Webb counties and also in the 

 east and south part of Maverick county. They consist of clays 

 and sandstones, and in places contain seams of lignitic coal and 

 occasionally some ledges of yellow calcareous rock. In general 

 appearance they differ but little from the sediments which make 

 up the Eagle Pass and the Pulliam formations. The boundary 

 between these two divisions were made out with fair accuracy by 

 a study of their physical characteristics. The sandstones of the 

 Tertiary age are more heavily impregnated with oxide of iron 

 than the Cretaceous beds, and this frequently gives them a deep 

 red, a brown, or a j'ellow color. They are also more variable in 

 their texture and contain coarser grains of quartz, that may 

 sometimes be known by their bluish white translucency. This is 

 especially true of the basal sandstones of the group. The clays 

 also are inclined to exhibit brighter distinctions of color, and from 

 layer to layer there are apt to be somewhat rapid changes from 

 one to the other of grayish white, yellow, red, purple, brown and 

 dark lead-gray. These colors are due to different degrees of oxida- 

 tion of the iron which they contain. 



The thickness of the Tertiary deposits it was not practicable 

 to make out except from such records as could be secured from 

 well-drillers in the Carizzo Springs artesian basin. There was no 

 opportunity to see any continuous set of samples of drillings, and 

 verbal descriptions by drillers are alwa^'s unsatisfactory. The 

 following section is given as an average of the results obtained. 

 It is quite evident that these clays and sandstones are even more 

 variable in their development than the Escondido beds. 



