IN THE UPPEK KIO GKANDE EMBAYilENT IM TEXAS. 79 



ing of the different rocks is not equally marked everywhere. 

 Ledges of sandstone occur in the clays wholly isolated from other 

 sandstones, and clay beds are nearly always present between the 

 sandy and the calcareous strata. The oyster shell breccias are 

 especiall}' irregular in their behaviour and occur in association 

 both with the clays and with the sands. They are also more apt 

 to run out and to undergo more rapid changes in thickness than 

 the other rocks. 



1. The basal sandstone is exposed in the banks on the bluffs 

 of the Rio Grande on the three surveys of Samuel Sanders. It 

 caps the bluffs which extend in a north and south direction from 

 three miles south of Eagle Pass to Arroj^o Seco, three miles north 

 of the city. North of this creek the escarpment formed by this 

 sandstone turns to the northeast, passing through surveys 6 and 

 4 in block 16 and through surveys 199, 200, 196, 197, 168, 169 

 in block 7, and rising to form the capping of a low hill known as 

 Chapotal hill in survey ] 70 in the same block. From this point 

 it can be followed for several miles to the south and east, but be- 

 yond that its course is doubtful. It should be added that this 

 sandstone is not a unit, but consists af different ledges separated 

 by strata of cla3^ At times it is broken up into two or three 

 members. In composition it varies from a siliceous sand to a 

 fairlj' pure limestone, containing a considerable amount of shell 

 fragments. It is mostly fine in texture, but there are occasional 

 seams of gravel. Ex.ogyra costata is sometimes seen in the clay 

 immediately below it. Eipple marks were noted in several places, 

 and fragments of bones of reptiles are not unkown. Barite is a 

 frequent mineral. 



The thickness of this sandy zone varies from thirty to one 

 hundred feet. 



2. The lower clay comes up to the surface in a swale of low 

 upland which follows the sandstone just described, on its east and 

 south side. The average width of this swale, or shelf, is two miles, 

 and the thickness of the clay is about 275 feet. In its upper half 

 the fossil shell Exogyra costata can be found almost wherever the 

 clay is exposed. By this fossil the belt was traced eastward from 

 Muerto hill to the John Foster survey, and from there northeast- 

 ward as far as to near Chilpotin tank on surA^ey 119 in block 7. 

 Fibrous barite was noted in several places in this clay. 



