469 
year or the range of variation is given in the tables; so also the differ- 
ence between the highest maximum and the lowest minimum reached 
during the whole period of observation. 
FLORAL DISTRICTS. 
Relatively to the differences in topographical and climatic conditions 
and in vegetation, the Black Hills may be divided into five districts: 
Foothills, Minnekahta Plains, Harney Mountain Range, 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 from that formation which is best represented within 
the region, as for instance, the Minnekahta Plains from the Red Beds, 
the Limestone District from the Limestone Plateau, the Harney Range 
from the granite crags. 
FOOTHILLS! AND SURROUNDING PLAINS, 
The foothills are capped by the comparatively hard cretaceous sand- 
stone. The plains outside the foothills are mostly covered by the 
overlying Miocene 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 Red Beds are exposed. Although 
the foothills constitute the outcropping of an older formation their 
flora is essentially the same as that of the surrounding table-lands, 
which extend as valleys far in among the hills. The flora depends 
more on 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 Hills proper. In facet 
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 ligh, 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 toa minimum. These characteristic plants may be divided into 
the following groups: 
(1) Very hairy plants, in many cases covered with a thick pannose 
pubescence. Such are: 
Eriogonum flavum. Senecio canus. 
Erioqgonum annuum. Senecio plattensis. 
Eriogonum multiceps. Evolvulus nuttallianus. 
Eriogonum pauciflorum. Filago prolifera. 
Astragalus gilviflorus. Spiesia lambertii sericea. 
Eurotia lanata. Artemisia frigida, 
Plantago purshii. 
1The western foothills are in Wyoming. The work was confined to South Dakota, 
and hence this includes only the eastern foothills. 
