4(59 



year or tlie ranjie of variation is given in the tables; so also tlie dift'er- 

 enee between the highest niaximnni and the lowest niinimnni reached 

 during the whole period of observation. 



FLOHAL DISTRICTS. 



Relatively to the diiierences in topographical and clinnitie conditions 

 and in vegetation, tlie Black Hills may be divided into tive districts: 

 Footliills, Minnekahta I'lains, Harney Mountain llange, Limestone 

 District, Northern Hills. 



These districts do not coincide with the outcroppings of the different 

 geological formations. They receive, however, their most prominent 

 physical features frcnn that formation which is best represented within 

 the region, as foi- instance, the IMinnekahta IMains from the Ked Beds, 

 the Limest(nie District from the Limestone Plateau, the Harney Kange 

 from the granite crags. 



KOOTllILLS 1 AND SCRIK >UM>I NU PLAINS. 



The foothills are capped by the comparatively hard cretaceous sand- 

 stone. The plains outside the foothills are mostly covered by the 

 overlying- Pliocene conglomerates and clays. In the canyons, along the 

 water courses, and in other depressions the underlying thin Jurassic 

 strata of clays and marls and the lied Beds are exposed. Although 

 the foothills constitute the outcropping of an older formation their 

 tiora is essentially the same as that of the surrounding table-lands, 

 which extend as valleys far in among the hills. The tiora depends 

 more ou meteorological conditions than on the geological formation. 

 As shown above, the annual rainfall at Rapid City, which is among the 

 foothills, is much less than that of the Black Llills i)roper. In fact 

 the conditions are much the same as in western Nebraska and eastern 

 Wyoming. It is a dry region, with most of the rain falling in the 

 spring, and a season of drought in July and August. A majority of the 

 plants peculiar to the high, dry plains of Nebraska, Wyoming, and 

 neighboring States were also found here. Most of these plants are 

 endowed with characters that in one way or another reduce the evapora- 

 tion to a minimum. These characteristic i)lants nmy be divided into 

 the following grou])s: 



(1) Very hairy plants, in many cases covered with a thick pannose 

 pubescence. Such are: 



Krioffoninii Jlaniin. >Senecio cainis. 



Erioiioninn unnuiim. Senecio plattenaiH. 



IJiio'joinan luiilticepx. Evolnilits VKllollianns. 



Erioijonitm pancijlorum. Filaijo prolifcra. 



AstriKjalns f/ilvijlorits. Spies'tu lamberili ni-rkea. 



Kurolia lanala. Arieiiiisia frigida, 

 Plantago pnrshii. 



'The westtTn foothills are in Wyoming. The work wan conliiied to Soulh Dakota, 

 and hence this includes only the eastern foothills. 



