Rocky Mountains. 489 



It had been a received opinion, among some of the geolo- 

 gists of the United States, that the Rocky Mountains were 

 not of primitive rocks; we had, hitherto, observed nothing 

 which could either confirm or invalidate this opinion. 



The great alluvial formation, which occupies the country 

 on both sides of the lower portion of the river Platte, is an 

 almost unmixed siliceous sand, in no manner distinguishable 

 from the debris of the sandstones of transition mountains. 

 Near the forks of the Platte, we first observed that the wa- 

 ters of that river bring down, among other matters, nume- 

 rous small scales of mica. This also is a constituent oi the 

 sandstones of the lower secondary or transition formations. 

 The fragments of unmixed and crystalline feldspar, which 

 now began to be of frequent occurrence, were considered as 

 the first convincing evidence of the primitive character of 

 the Rocky Mountains. These fragments of feldspar, we 

 believed, could have been derived from no other than primi- 

 tive rocks. 



During all the day on the 23rd, we travelled along the 

 south side of the Platte, our course inclining something more 

 towards the southwest than heretofore. 



Intermixed in the narrow fringe of timber, which marks 

 the course of the river, are very numerous trees, killed by 

 the action of the beaver or by the effects of old age, their 

 decorticated and bleached trunks and limbs strongly con- 

 trasting with the surrounding objects, many of them render- 

 ed doubly intere-ting by affording a support to the nests of 

 the bald eagle, elevated like a beacon in the horizon of the 

 traveller. 



Large herds of bisons were seen in every direction; but 

 as we had already killed a deer, and were supplied with meat 

 enough for the day, none of the party were allowed to go in 

 pursuit of them. Prickly pears became more and more 

 abundant as we ascended the river, and here they occurred 

 in such extensive patches as considerably to retard our pro- 



