THE CRETACEOUS SEDIMENTS. 



THE DEVIL'S RIVER LIMESTONE. 



This name is here applied to the limestones which are found 

 exposed on Devil's river. They include what is known in central 

 Texas as the Edwards and the Georgetown limestones. The 

 upper hundred feet, or less, coiTespond to the latter, and the 

 lower four hundred feet are the equivalent of the greater part of 

 the former. They are not separated hy any well marked horizon 

 of change, but merge gradually into each other. 



This rock consists of mostly thick ledges of a white or gray 

 limestone of quite pure composition. It varies from a moderately 

 coarse grained to an almost compact and structureless texture. 

 The ledges are mostly thick, occasionally' measuring as much as 

 ten or even twenty feet. But there are two horizons where the 

 seams of stratification are more closely laid-, and these occur at 

 from seventy-five to a hundred and fifty feet, and again at from 

 two hundred and twenty-five to three hundred feet below the top 

 of the formation. This alternation of horizons of more and less 

 enduring rock causes the bluffs on either side of Devil's river to 

 vary in their declivity at different heights. The cliffs of the more 

 enduring divisions are more difficult to scale and often impass- 

 able, while the slopes are less abrupt on the exposed edges of the 

 less heavily bedded strata. In the same way we find the valley of 

 the river more open at points where it runs over the softer beds, 

 as above the mouth of .San Pedro creek, at Pafford's crossing, be- 

 low the old Newman's ranch, and again above the old Ames's 

 ranch, while it takes on the true character of a narrow canyon 

 when cutting on the harder ledges, as it does for several miles 

 north of the mouth of San Pedro creek, north of Deadman's can- 

 yon and above Newman's ranch. For the same reason the roads 

 leading down to the river from the plateaus through tbe arroyos 

 from either side are least satisfactory, where these canj'ons lead 

 over the heaviest limestone ledges. In fact, it is these ledges which 

 make most of the lateral canyons impassable. In surveys 26, 27, 



