^ " Expedition to the 



the nostrils. Scarce any two of either species are precisely si- 

 milar in colour, but the markings are permanent. Both species 

 possess, in a slight degree, the power of varying the shades 

 of colour. . We could find no conspicuous difference mark- 

 ing the different sexes in the species with long spines; the 

 other we have not had sufficient opportunity to examine. 



Aug. 2d. The rain w^hich had fallen during great part of 

 the preceding day and night, had considerably raised the 

 water in the small creek, on which we were encamped. At 

 sunrise we collected our horses, and proceeded down the 

 valley, the direction of our course, being south, 80** east. At 

 the distance of two or three miles we found the valley much 

 expanded in width, and observed a conspicuous change in 

 the sandstone precipices, which bound it. This change is 

 the occurrence of a second variety of sand-rock, appearing 

 along the base of the cliff, and supporting the slaty argillace- 

 ous stratum above described. These rocks have the same 

 relative position, and nearly the same aggregate elevation, 

 as the two very similar varieties in the valley of Purgatory 

 creek: indeed the conclusion, that they are the continuation 

 of the same strata, can scarcely be avoided. The lower- 

 most or red sand-rock, is here very friable and coarse. Its 

 prevailing colour is a yellowish gray or light brown. It 

 often consists almost exclusively of large rounded parti- 

 cles of white or transparent quartz, united by a scanty ce- 

 ment, which usually contains lime, and sometimes, but not 

 always, oxide of iron. In some instances the cement seems 

 to be wanting. Its stratifications are very indistinct, com- 

 pared to those of the gray sandstone, and like them disposed 

 horizontally. 



On entering the wider part of the valley, we perceived be- 

 fore us, standing alone in the middle of the plain, an im- 

 mense circular elevation, rising nearly to the level of the 

 surface of the sandstone table, and apparently inaccessible 

 on all sides. On its summit is a level area of several acres 



