Rocky Mountains. 477 



phenomenon was so often observed that we were induced to 

 attribute it to the operation of the same local cause, which 

 in the neighbourhood of the sea, produces a diurnal change 

 in the winds, which blow alternately to and from the shore. 

 The Rocky Mountains may be considered as forming the 

 shore of that sea of sand, which is traversed by the Platte, 

 and extends northward to the Missouri, above the great bend. 



The rarefaction of the air over this great plain, by the re- 

 verberation of the sun's rays during the day, causes an as- 

 cending current, which is supplied by the rushing down of 

 the condensed air from the mountains. Though the sun's 

 rays in the middle of the day, were scorching and extremely 

 afflictive to our eyes, the temperature of the air as indicated 

 by the thermometer, had hitherto rarely exceeded 80° Fah. 



In the forenoon we passed a range of hills more elevated 

 than any we had seen west of the Missouri. These hills cross 

 the Platte from north to south, and though inconsiderable in 

 magnitude, they can be distinguished extending several miles 

 on each side of the river. They consist principally of gravel, 

 intermixed with small water-worn fragments of granite and 

 other primitive rocks, but are based on a stratum of coarse 

 friable sandstone, of a dark gray colour, which has been un- 

 covered, and cut through by the bed of the Platte. 



This range may perhaps be a continuation or spur from 

 the black hills mentioued by Lewis and Clark, as containing 

 the sources of the Shienne, and other tributaries to the Mis- 

 souri, at no great distance to the north of the place where 

 we now were. 



At evening we arrived at another scattering grove of cot- 

 ton-wood trees, among which we placed our camp, immedi- 

 ately on the brink of the river. The trees of which these in- 

 sulated groves are usually composed, from their low and 

 branching figure, and their remoteness from each other, as 

 they stand scattered over the soil, they occupy, revived 

 strongly in our minds the appearance and gratifications re- 



