298 GEOLOGY. 



areas. They are also found over a limited area north of the Nio- 

 brara. Hayden (Report for 1S70, p. 10S) estimates the area of the 

 sand-hills at about 20,000 square milas. From exploring the same 

 region, I should not estimate them as so extensive, unless the fact 

 be kept in mind that they are not continuous over the whole region. 

 They are indeed found all the way for 100 miles west from the 

 mouth of Rapid River, but in many places from eight to twenty- 

 miles south of the Niobrara there are spots where the soil seemed 

 to be a mixture of Drift and Loess, and of high fertility, as was in- 

 dicated by the character and rankness of the vegetation. Some- 

 times these hills are comparatively barren, and then again they are 

 fertile enough to sustain a covering of nutritious grasses; so that 

 this region is by no means the utterly barren waste that it is some- 

 times represented to be. It has been a favorite range for buffalo, 

 and still is for antelope and deer; and, judging from their condition, 

 the conclusion would be natural that this region could be used for 

 stock-raising. In fact, already large herds of cattle are kept here. 

 A great deal of the vegetation is peculiar to sandy districts. Some 

 of the hills seem to have their loose sands held together by the 

 Ucca ciTigustifolia, which sends its roots down to a great depth. It 

 probably marks a certain stage in their history. After this plant 

 has compacted and given to the sands organic matter, the grasses 

 come in and partially clothe the hills. The materials of these sand- 

 hills are almost entirely sand, pebbles, and gravel, of varying de- 

 grees of fineness. The sand always predominates. Occasionally it 

 is more or less modified by the presence of other materials, such as 

 lime, potash, soda, alumina, and organic matter. These hills are 

 in some places stationary, and so covered by vegetation that their 

 true character is not suspected until closely examined. In other 

 places again, especially in portions of the Loup and Niobrara region, 

 they are so loosely compacted that the wind is ever changing their 

 form, and turning them into all kinds of fantastic shapes. The 

 most common appearance is that of a plain, undulating or hilly re- 

 gion, covered with conical hills of drifting sands. The smaller ele- 

 vations frequently show a striking resemblance to craters. One 

 such curious hill I found south of the Calamus, where the crater- 

 like basin seemed to be compacted at once, and grown over with a 

 species of wire grass. With the increase of rainfall and vegetation, 

 the remodifying effects of the winds disappear. 



