LO^VER RIO BRAVO TRIBUTARIES. 41 



Sierras or mountain ranges, formed of igneous and metamorphic stratified and unstratified 

 rocks border, on the Mexican side, tlie cretaceous basin of the Rio Bravo. To these that side of 

 the river is indebted for running water in the shape of several large, clear-water streams. 

 Wliere there is hardly one running stream on the Texan side, on the Mexican are six, which 

 carry no inconsiderable quantity of water in the Rio Bravo. 



Besides the small streams, very changeable in their supply of water^ the following are re- 

 markable for their constant flow : 



Escondido, near the head of which is situated the town of San Fernando, has its mouth about 

 three miles below Eagle Pass. 



Las Cavezeras, heading in the immediate vicinity of the Escondido, with the little town of 

 San Juan de Allende and Nava near its source, empties some thirty miles above the destroyed 

 rancho Palafox. 



The Salado, carrying what may be considered here a large body of water, which is supplied 

 by several large branches from near Santa Rosa, (Sabine, Alamo, and others,) commingles its 

 waters with the Rio Bravo some eight miles below Guerrero, and nearly opposite Redmond's 

 rancho. 



Alamo, having its source in, and bringing its waters down from, a more southern portion 

 of the Mexican Sierra, falls into the Rio Bravo near Mier. 



The San Juan, gathering its waters in still more southern portions of the Sierras which form 

 the highlands of the States of Coahuila and Nueva Leon, empties just below Camargo. 



There is almost no running water on the Texan side from the beginning of the green sand 

 belt down to the mouth of the Rio Bravo, if we except some few sluggish, half-underground, 

 trickling creeks. If it were not, however, for their subterranean course, which prevents entire 

 evaporation, this region would be wholly destitute of water. 



The state of water near the mouth of these creeks is always affected by the rise or fall of 

 the river, and also by the hygrometric precipitation of the atmosphere, however small quantities 

 this latter may contribute. 



These peculiar water-courses form a characteristic feature of the country through which they 

 run ; and their thorough knowledge gives advantages to the natives, by which they elude the 

 pursuit of the white man, and render his efforts of no avail. 



THE ISLANDS. 



Another peculiarity of the portion of the Rio Bravo that winds through these cretaceous 

 regions is the frequent occurrence of islands, various in size and appearance. 



Whilst in the upper part of this cretaceous main the size of the islands is reduced to that of 

 mere bars and reefs, (bare, or covered with rush and cane,) further below their absolute height 

 above the water increases, and consequently the number of vegetable forms growing upon them. 



As nearly all the islands are formed by alluvial deposits on stratified rocks, the layers of 

 which they are composed assume or are arranged in a strictly horizontal position. As a matter 

 of course, therefore, the down-river end usually rises high above the water, while the up-river 

 end always appears partially covered with muddy and gravelly deposits and driftwood and the 

 like that the river is continually carrying down. 

 fiM 



