Rocky Mouiitains. 129 



founded on the distribution of certain remarkable plants. The 

 grt-at cylindric cactus, the American colycinth, (Cucumis 

 perennifi,) and the small-ltaved elm, might be used in such 

 an attempt, but it is easy to see that the advantages resulting 

 from it, woiild be for the most part imaginary. 



Discussions of this sort have been much insisted on, and 

 may be important as aiding in the geography of climates and 

 soils, but can afford little assistance to topography. 



The geological features of the region under consideration, 

 afford some foundation for a natural division, but this divi- 

 sion must be so extremely general, as to afford little satis- 

 faction. We could only distinguish the red sandstone, the 

 argillaceous sandstone, and the -trap districts, and though 

 each of these have distinctive characters, not easy to be mis- 

 taken, they are so irregular in form and position, as to be in 

 no degree adapted to aid in the description, and identifying 

 of particular places. On the contrary it is to be regretted, 

 there are no established points, to which we might refer, in 

 communicating what we have observed of the position of 

 these formations, and indicating the particular localities of 

 some of the valuable minerals they contain. 



The red sandstone, apparently the most extensive of the 

 rocky formations of this region, shows, wherever it occurs, 

 indications of the presence of muriate of soda, and almost 

 as commonly discloses veins and beds of sulphate of lime. 

 This substance had been growing more and more abundant, 

 since we left the region of the trap rocks at the sources of 

 the river. It was now so frequent as to be conspicuous in all 

 the exposed portions of the sand-rock, and was often seen 

 from a distance of several miles. It occurs under various 

 forms; sometimes we met with the most beautiful selenite, 

 disposed in broad reticulating veins, traversing the sand- 

 stone. The granular and fibrous varieties, whose snowy white- 

 ness contrasts strongly with the deep red and brown of the 

 sandstone, are sometimes seen in thin horizontal laminae, or 



VOL. 11. 17 



