78 EXPLORING EXPEDITION FROM SANTA KK 



crnniti, Nautili, and fish-scales. The Piedra Parada, a well-known landmark, which 

 has conferred its name upon the Rio Piedra, is a chimney-like column of rock, rising 

 with its base to the height of eight or nine hundred feet above the surrounding conn- 

 try. It is itself composed of the Upper Cretaceous sandstone, resting upon a base of 

 dark shales, (Middle Cretaceous,) and is a remnant of a- plateau now almost entirely 

 removed by urosiou. 



The valley of the Rio Piedra is, in physical aspect and geological structure, the 

 counterpart of that of the Rito Blanco, already described; the only noteworthy point 

 of difference being that the Upper Cretaceous sandstone here contains beds of lig- 

 nite of considerable thickness, though of limited lateral extent. The disturbances 

 which the Cretaceous strata have suffered in this region are very well shown 

 in the immediate vicinity of our camp on the Piedra. On the east side of the 

 river the plateau, with which the Piedra Parada was once connected, stretches 

 away south for several miles, with a rapid dip in that direction. On the west 

 side is a mesa of which the .cut-edge rises abruptly from the margin of the 

 stream to the height of over twelve hundred feet. This me.sa is composed of 

 precisely the same materials as that bounding the valley on the east, but its strain 

 are almost perfectly horizontal. Just above our camp this mesa is broken short 

 off by a lino of fracture running nearly east and west; its component rocks, set nearly 

 on edge, forming a wall which rises to the height of eighteen hundred feet above the 

 river. Between this disrupted mass and the mesa from which it has been broken is a 

 low pass through which our trail ran. 



Soon after leaving the Rio Piedra we began to ascend rapidly and soon rose on 

 to a high divide between the Piedra and Rio de los Pinos. This divide seems to bean 

 axis of elevation, running southward from the mountains between the Piedra and Pinos. 

 It is composed of Cretaceous rocks irregularly broken up, generally inclined at a high 

 angle. It has an altitude of about sixteen hundred feet above our camp on the I'iedra. 

 The view from its summit is peculiarly grand and interesting; eastward the Xavajoand 

 Sau Juan mountains bound the horizon, terminating southward in the Cerro del Nav- 

 ajo. On the south and west sides of these ranges the fixers we have recently passed 

 take their rise, and their valleys may in part, be followed by the eye. None of the 

 summits visible in this direction are covered with perpetual snow, but all bear patches 

 in the higher valleys. 



Nearer us than the mountains was a labyrinth of hills, spread out before us as on 

 a. map. These seern to have been generally formed by a series of breaks in the Cre- 

 taceous rocks, and, so far as we could see, are all composed of these materials. 



Toward the west our view was quite different. In that direction we looked 

 down on . what seemed an extensive plain, extending northward to the base of the 

 Sierra de la Plata, now for the first time distinctly seen, and on the south bounded by 

 high mesas, through an opening in which, the gap of the Rio las Animas, was visible 

 the high chain of the Carriso Mountains south of the San Juan This plain before us 

 we subsequently found to be a basin-like space, lying between the mountains on the 

 north and the table-lands south; a depressed area, completely inclosed in high 

 lands, except where these are cut by the narrow- gorges through which the waters of 



