58 GEOLOGY. 



In order to reach the lower basin in tlie conrse of the Eio Grande, beyond the San Carlos 

 canon, you have to make an extensive detour, and pass again up the San Carlos creek about ten 

 miles to the " Oid Presidio of San Carlos." This now deserted adobe structure is situated on 

 the eastern side of the valley, occupying gravelly table-land, and overlooking the alluvial 

 bottom, which shows the remains of former cultivation in extensive lines of irrigating ditches. 

 The cultivation carried on by the present inhabitants of San Carlos is confined to the upper 

 part of the valley. These cultivated fields extend some five miles, and present a rich belt of 

 alluvial soil, abundantly watered and consequently fertile. 



On leaving the San Carlos valley, we pass by a southeast course over an upland, gently 

 undulating plain, set oiF by occasional rocky knolls, and encircled by mountains mostly of 

 volcanic formation. Our course thence, inclining more to the northeast, leads over broken 

 swells of the limestone range pertaining to the canon above described. Thus, by a series of 

 steep descents, the lower basin of tlie Rio Grande is gained at a point some distance below the 

 mouth of the San Carlos canon, and nearly opposite the range of the Chisos mountains. 



The general character of the valley here presents the usual features of gravelly table-land, 

 flanking a narrow alluvial belt along the winding course of the river. The bottom-land here 

 is again bordered by cotton-wood and willow growth. 



Towards the exit of the river, from the San Carlos canon above, the general aspect of the 

 valley is modified by irregular outbursts of eruptive trap rocks, confusedly alternating with 

 volcanic breccias. 



The general course of the river through this basin is easterly ; the bottom-land is of limited 

 extent, and generally barren. At several points here are seen Indian fords and broad trails 

 leading from upper Texas into Mexico. These beaten paths are unmistakable indications of 

 the route pursued by the Camanches on their extensive foraging expeditions. 



These routes, both to the north and south, are comparatively open, and are apparently deter- 

 mined by tlie dej^ressions that occur in the elsewhere uninterrupted line of mountains. 



The continuation of our land route down the river compelled us to cross, with our pack train, 

 at one of the Indian fords called " Fado Fleclie," thence taking along the Texan side to turn 

 the spur of the mountain range forming the San Vincente canon. 



This range is exclusively composed of cretaceous limestone, similar in texture to that in the 

 San Carlos caiion above. It differs from that, however, in being of less height and extent, and 

 in showing, in place of a regular western dip, a distinct anticlinal axis, the dip being quite 

 abrupt on either side. Cretaceous fossils, identical with those found at San Carlos, serve 

 2)lainly to characterize this formation. 



The canon ot San Vincente is very abrupt and of considerable height ; the ridge adjoining is 

 also very broken, exhibiting steep descents and branch chasms, which rendered the survey 

 extremely arduous. Tlie passage through this cannon was accomplished by our India-rubber 

 boats, one of which was, however, capsized in shooting a sharp rapid. On emerging from this 

 range, the Rio Grande opens on the San Vincente basin, in which, on the Mexican, side is 

 situated the now deserted Fort San Vincente. 



This basin differs from all others yet seen in being exclusively formed of low ledges of a 

 dark-color d stratified rock, showing low bluff ranges 30 to 50 feet high. These ridges have a 

 very uniform dip of 15° to the northeast ; they occupy in great measure the place of the usual 



