THE FAYETTE BEDS. 57 



and rusty iron pyrites. At the lower end it is overlaid by similar beds, but 

 somewhat harder and more sandy. Frequently small white calcareous con- 

 cretions, and sometimes large clay indurations with veins of crystalline calcite, 

 are found. Three miles below are found similar beds just above the water 

 level, and overlaid by fifteen feet of sandstone, with concretions, fragments 

 of worn silicified wood, and a few broken pieces of an oyster. These latter 

 have the appearance of being derivative and not indigenous to the bed, as 

 they are much rounded and rolled, and were very probably derived, during 

 the deposition of the enclosing clays, from the great oyster beds of the strata 

 about Roma. Two miles below this are seen similar beds, but with no clay, 

 the soft and indurated layers alternating with each other. The dip is 2 de- 

 grees north 20 degrees east. This, however, is a local variation due to the 

 causes explained on pages 16 and 45. Nine miles above Rio Grande City 

 are seen similar sands with silicified trunks and branches of trees. The sands 

 have the characteristic grains seen in the Fayette Beds on the Brazos and 

 Colorado. 



The town of Rio Grande City (Ringgold Barracks) is situated on a bluff of 

 hard white clay, rising some fifty feet above the river, and indurated into a 

 substance of a chalky consistency, though, chemically, it is only very slightly 

 calcareous. It probably represents the light green clays of the Fayette Beds, 

 and has become indurated by exposure to heat in a dry climate. The effect 

 of such agencies would also account for its white appearance, as the charac- 

 teristic pale green color of these clays is doubtless due to their hydration. 

 The bed shows a highly conchoidal fracture, contains iron pyrites, and is 

 much jointed. These joint cracks are frequently filled by veins of smoky 

 quartz one-eighth inch to one inch thick, often showing a globular surface. 

 The bluff extends along the river for half a mile below the town, and two 

 hundred yards above it. Beyond these limits it disappears under the gray 

 river silt. 



Below Rio Grande City we pass through low alluvial banks for a distance 

 of twenty-two miles. About a mile back from the river at this point, and 

 a short distance below Las Cuevas, is a bluff of semi-hardened sharp sand, 

 with lenticular seams of coarse sand and siliceous pebbles one-sixteenth inch 

 to one-half inch in diameter, also white calcareous nodules one to three inches, 

 and seams of calcareous gray clay. The bluff is fifty feet high, the upper 

 ten feet being a quaternary conglomerate of river pebbles cemented in a white 

 calcareous matrix. At a point ten miles above the Texas town of Hidalgo, 

 or Edinburg. in Hidalgo County, is a low ledge rising one foot above the 

 water and composed of Fayette sands. A similar outcrop is seen at the 

 water edge at Reynosa, in Tamaulipas, and directly opposite Hidalgo. This 

 outcrop reaches only two feet above the water edge, and overlying it and 



