30 • GEOLOGY. 



that can be procured throughout the arid regions bordering the upper portion of the cretaceous 

 basin of the river. Whilst the running water in these dry beds can find its way only by a sub- 

 terranean passage through the holes and fissures presented by this formation, pools and small 

 ponds of 150 to 200 feet in length and breadth occur in the cavities formed in the solid masses 

 of rock. The valleys of the rivers Bravo, San Pedro or Devil's river, and Pecos, resemble each 

 other in this respect. 



BOTTOM-LAND. 



Soil suitable for cultivation is scarcely seen in these river bottoms. When small slips of it 

 appear, it is confined to places where a projecting rock or a deposit of mud and drift-wood offers 

 some protection against the violence of the currents everywhere present in these rivers. 



Such patches of bottom-land offer the only shelter and home for the growth of trees, consisting 

 almost exclusively of live oak, hackberry, pecan tree, ash, and some two or three species of rhus. 

 The country embraced by this formation is a waterless region^ with a barren and rugged surface. 

 There is but one constantly running tributary of the Eio Bravo between the mouths of the 

 Pecos and San Pedro, a distance of 40 miles. The waters of this tributary, of a blue crystal- 

 like transparency, boil out in a deep chasm from beneath a solid mass of limestone, and pour a 

 rapid and full current into the river, but a few paces distant, through dark green shades of flowery 

 and fragrant thickets that line its bed. Its solitary beauty, amid the barrenness and unbroken 

 silence of the surrounding wilderness, suggested a fairy creation, and suggested the name of 

 " Fairy Spring" to this enchanting stream. 



Other ravines or '^ rock creeks" aff'ord at times a small stream of clear running water. In 

 their rocky beds occur here and there a series of water holes and small ponds, either isolated or 

 connected only by a trickling run of water. There are several creeks of this character, especially 

 in the vicinity of the Pecos ; among which may be mentioned Painted Gallery, Nine-tailed Cat, 

 Oak creek^ and Fox-hole. 



ANTEDILUVIAL DUNES. 



Groups of hills and low ridges, from 80 to 100 feet in absolute height, appear in different 

 localities on the table land of this region. Fossil remains are found on their slopes, and show 

 that they also belong to the cretaceous formation, and constitute its last link. These hills may 

 be considered as accumulations of cretaceous debris, preceding tertiary strata. The irregularity 

 of the line of their direction, and their unmistakeable parallelism with the water-courses, lead 

 to the conclusion that they really once bordered the submarine currents of a vast cretaceous sea, 

 of which the section of country towards the mouth forms a part. Thus we are able to trace 

 these antediluvial dunes on both sides of the river, and all its tributaries, not only in its upper 

 part, but even as far down as the point where the cretaceous ridges come into view. 



The Rio San Pedro forms a kind of geological boundary, and seems to have some close rela- 

 tion to other physical peculiarities of the adjoining region. Some changes are here perceptible 

 in the fauna, flora, and ineteorology ; to speak of which, however, is foreign to the matter in 

 hand. 



UPPER METAMORPHIC LIMESTONE. 



The limestone below the mouth of the San Pedro or Devil's river does not form such solid 

 masses as that above. The high table-lands, already described, change into a more rolling, 



