LOWEli KIO BKAVO. 3<J 



FOSSIL BONE. 



The lower portion of the green sand belt reaches as far down as the vicinity of Reynosa, 

 which is about 200 miles from the Gulf. Here the last cretaceous ridge of hills approaches the 

 valley of the Rio Bravo, though no rocks are exposed on its banks. 



Tertiary strata seem, however, to occur alternately throughout the whole country. Many 

 bluffs and ridges may be referred to this formation ; for a fossil bone was taken out of a blulf 

 near Camargo, on the Rio San Juan, and was in the possession of a resident of Eagle Pass, who 

 stated that many such interesting remains of fossil fauna were to be found in the same and 

 similar localities. » , 



FOSSIL WOOD. 



As to the presence of tertiary strata in the valley of the Rio Bravo, it would be proper to add 

 here another suggestion based upon the frequent occurrence of fossil wood ; by a close examina- 

 tion of which, some three or four genera or species might be classified. 



The mouth of Elm creek is particularly distinguished for the abundance of fossil wood, which is 

 usually found here scattered about as if brought down from some higher localities ; yet pieces of 

 trunk and branch sometimes were so arranged as to lead one to suppose that they once belonged 

 to a tree on the spot, which had just fallen to the ground. Not far from this place, and very 

 near the layers of lignite coal before spoken of as lying adjacent to Eagle Pass, the trunk of a 

 fossil tree was discovered in one of the innumerable ravines cutting the borders of the Rio Bravo 

 valley, 2^ feet in length by 15 inches in diameter. The largest pieces of fossil wood, however, 

 were noticed in the environs of Santa Rosa; it abounds especially at the foot of the table-land 

 ridges, running out in a northerly direction from the main ridge of the argentiferous mountains. 

 These flat ridges, called by the inhabitants " Las Mesas" or "Lomas," belong to the cretaceous 

 system, and are covered with strata, from 20 to 30 feet thick, of basaltic rock and trap or basaltic 

 tuff. 



So far as could be seen with the naked eye, there is not much difference between the texture 

 of this fossil wood and of that near Eagle Pass and at other places along the valley of the Rio 

 Bravo. Most of this fossil wood possesses apparently the structure of the palm-wood. The 

 color of the specimens from Santa Rosa, however, is quite different from the rest. They show 

 a dark reddish brown, much like the basalt in their neighborhood, placed on the top of the 

 mesas. It seems highly probable that this wood may have been at some time in contact with 

 these igneous and eruptive rocks, and that this red brown color was imparted to it by a certain 

 proportion of oxide of iron. 



The larger pieces of this fossil wood have been carried to town to serve as corner-stones. 

 Some of these measured from 3 to 3^ feet in length by from 12 to 18 inches in diameter. The 

 large size of the Santa Rosa specimens, compared with those found in the valley of the Rio 

 Bravo and the surrounding country, led us to conclude that the former were not brought from 

 so great a distance as the latter; in fact the former could not liave originated far off', because 

 the tertiary strata, which are undoubtedly their home, cannot be sought for on the top of the 

 Santa Rosa mountains, bordering as these do the cretaceous basin of the Rio Bravo. 



Whether all the fossil woods of the various localities mentioned are endogenous or not, wo are 



