THE LOWER RIO BEAVO. 43* 



Besides the presence of extensive fossil oyster-banks in its vicinity, there is below its mouth, on 

 the Mexican side, a most rugged and naked spot, which presents the appearance of having just 

 been torn to pieces by some destructive freak of nature. The scene of this apparent catastrophe 

 now constitutes a wjde flat basin, which is arid and literally cut up in every direction by ravines 

 and rills. On the borders of these ravines, which rua to a common centre and form a pool of 

 water known as the "Sulphur Spring," are to be found a sulphur-colored earth, probably 

 alumn and sulphur. The mineral water of this spring just now under examination tastes 

 strongly of sulphurated hydrogen gas, and after standing for some hours in a vessel, collects a 

 black deposit on the bottom. This water is celebrated in the country around for its medicinal 

 properties. When the American army was camped near here during the Mexican war, it was 

 much used on the recommendation of the surgeons. 



Here, within a distance of about 2| miles, several water-courses empty into the Rio Bravo, 

 and form by their deposits a large island, which, as a result of this united action, was named 

 "Los Ajuntas ;" besides the Alamo, already mentioned, these tributaries are the Sulphur 

 Spring, the Saladito, and Arroyo Hondo. 



THE FLORA. 



The flora of both the green sand belts show, as to the number of genera and species, various 

 additions, and a more luxuriant development when compared with the vegetable life of the 

 regions above. 



As instances, the Huisache and Guaxillo, and other plants, all of the Mimosa family, grow- 

 ing above only as arborescents, gradually rise in these belts, and further down in the valley, to 

 the size of conspicuous trees, often forming dense copses in the old beds and on the banks of the 

 river. Of the several genera added, especially noticeable, are the Coma tree, the Nacavites, the 

 Anacua, and Sabina ; the latter two seem to have been brought in by the Rio Salado. 



The appearance of the Cypress (Taxodium) Sabina of the Mexicans makes quite a striking 

 change in the jihysiognomy of the country. This water and ruck-loving tree often appears 

 right in the middle of the swiftest currents of the river, where having taken hold of some rocks 

 on the bottom, it boldly defies the force and action of the water. These cypresses on the Rio 

 Bravo, commencing at the month of the Salado, extend as far down as Beaver Islands, just 

 below Roma, where they make their last appearance in the channel on the Mexican side, footing 

 in water from 14 to 15 feet in depth. 



THE COLLUVIAL BELT. 



After passing the lower green sand belt, (geographically speaking,) which has its lowest 

 limits in the vicinity of Reynosa, as already stated, the coUuvial commences and extends down 

 to the coast. Under the term " colluvial" we include both the diluvial and alluvial deposits ; 

 these are irregularly and somewhat alternately arranged — the result of the oscillatory action of 

 the salt waters from the sea-side, and the antagonistic and downward force of the fresh waters. 



It is on account of this irregular distribution that the colluvial belt cannot be properly sub- 

 divided into a diluvial and an alluvial portion. Topography and, in some degree, botany, may, 

 however, aid in examining, in its subdivisions, this lowest belt of the cretaceous basin of the Rio 

 Bravo. 



