S99 Expedition to the 



perceptible inclination, and present appearances, which indi- 

 cate their deposition to have been nearly contemporaneous 

 to that of many of our coal formations. It has already been 

 suggested, that this range may probably be a continuation of 

 the Cotes Noire^ or Black Hills, said to contain the sources of 

 the Shienne, the Little Missouri, and some branches of the 

 Yellow Stone.* 



Beyond these inconsiderable hills, the surface again sub- 

 sides nearly to a plain. The fine and comparatively fertile 

 sand which prevailed to the east of the range, is exchanged 

 for a gravel, consisting of rounded granitic fragments, vary- 

 ing in dimensions from the size of a six pound shot to the fi- 

 nest sand. This great mass of granitic fragments, evidently 

 brought down by the agency of water from the sides and 

 summits of the Rocky Mountains, slopes gradually from 

 their base, appearing as far as our examinations have ex- 

 tended, in some measure to correspond in magnitude to the 

 elevation and extent of that part of the mountains opposite 

 which it is placed. The minute particles derived principally 

 from the quartzy portions of the primitive aggr-egates, being 

 least liable to decomposition, have been carried to the great- 

 est distance, and now form the soil of the eastern margin of 

 the great sandy desert. The central portions are of a coarser 

 sand, intermixed with some particles of feldspar and mica, 

 and minute fragments of hornblend. Nearer the mountains, 

 pebbles and boulders become frequent, and at length almost 

 cover the surface of the country. 



That the great Sandy Desert has resulted from the wear- 

 ing down of the mountains, both before and since the retiring 

 of the ocean, will not be thought improbable, if we consider 

 that the materials composing both regions are similar in kind, 

 —that the granitic soils of the plain are precisely such as 

 would result from the disintegration of the rocks now exist- 

 ing in the mountains, and that the numerous deep ravines 

 and water- worn vallies, traversing the mountains in various 

 directions, indicate such a change as is here supposed to 

 have happened. 



It is probable many parts of this extensive desert may dif- 

 fer from that traversed by the Platte, in having the surface 

 more or less covered with horizontal strata of sandstone, 

 and conglomerate instead of loose sand and pebbles. Many 

 appearances indicate that a formation of this kind formerly 



* Lewis and Clark's History, vol. i, p. 183. 



