228 E. T. DUMBLE GEOLOGY OF THE RIO GRANDE. 



found a series of sandy clays capped by sandstones, with an indurated 

 glauconitic layer containing small oysters and other fossil forms. This 

 sandstone is the same as that capping the hills at Eagle Pass and is the 

 lowest stratum of the Escondido beds.* Passing down the river this 

 sandstone thickens and shows ripple markings in places, and has an 

 apparent dip of at least 2°. The exposure is a mile in length, and con- 

 sists of sandstones alternating with clays. Fossils are very abundant 

 and well preserved, consisting of Ammonites (Placenticeras), oysters and 

 other bivalves, and several gasteropoda. Similar exposures continue for 

 4 miles below Eagle Pass. Above these come other blue clays and thin 

 sandstones with many oysters. 



At Fortress bluff, 6 miles below Eagle Pass, the exposure has a height 

 of 60 feet, and is composed of sandstones with seams of sandy clay 

 interstratified. The first of the great oyster beds occurs here in strata 

 six inches to a foot in thickness. Similar exposures continue to the 

 bluffs 10 miles from Eagle Pass. The sandstones at this locality are 

 highly calcareous and contain several beds of oyster shells. 



From this point to the falls of the Rio Grande, just above the Webb 

 county line, the exposures are but repetitions one of another — brown. 

 buff, blue, or green clays, with sandstones, sometimes friable and some- 

 times so indurated as to be semi-quartzites. Abundant fossils, consisting 

 of Ammonites (Placenticeras), oysters and gasteropoda, are found. The 

 rapids (or falls of the Rio Grande), which continue almost to the line 

 between the two counties, are formed by the edge's of some of these 

 ammonite-bearing beds as they pass below water level. From this point 

 to Webb bluff, a distance of 3 miles, no fossils were found ; but there was 

 no change in the lithologic character of the rock materials, nor could 

 the clays at the base of the Webb bluff section be distinguished in any 

 way from those observed at the rapids above. 



c 



Webb Bluff Section. 



Feet. 



(xiavel , 



Sandstone, white and glistening, with mica and some little iron; calcareous 

 sandstones; clay with cannon-ball concretions; and small seam of gra- 

 hamite MO 



Greensand marls with many Tertiary fossils; nodnles of carbonate of lime; 

 specks of glauconite 7 to 8 



Stiff, plastic dark greenish or bine clay, jointed 10 



We have therefore only •'{ miles in which there can be any room for 

 deposits intermediate between strata containing fossils of recognized and 



♦From Mr. Owen's examinations I learn that this is a very persistent bed throughout Maverick 

 county and is easily recognizable. It is about rive hundred feet above the coal seam, and is there- 

 fore valuable as a definite horizon from which to work in prospecting for the coal. 



