352 
From the foregoing the following facts may be gleaned: 
1. In the lichen flora of the Black Hills the rock lichens 
greatly predominate and have a more general distribution, the 
earth forms coming next. 
2. This lichen flora is essentiaily northern in character. 
3. It is more closely allied to the flora of the Rocky Moun- 
tains than to that of Eastern Nebraska and Dakota and hence, of 
course, to that of the Mississippi Valley. 
4. That though intermediate between the lichen flora of the 
Rocky Mountains and that of the Mississippi Valley, it is not so 
distinctly so as the geographical position of the Black Hills region 
might lead us to expect. oes 
The explanation of these conclusions can be found, undoubt- 
edly, in the climate and geological make-up and history of the 
Black Hills region. The wide stretch of prairie, with its hot dry 
winds, lying between the Black Hills and the Lower Missouri 
Valley, as well as the Mississippi Valley proper, is not favorable 
to the development and growth of tree-loving species, even when 
plenty of trees are to be found. On the other hand, saxicoline 
and terricoline species survive because better protected and more 
likely to get enough moisture to retain life even in times of exces- 
sive drought. This condition of things would break the connec- 
tion between the Black Hills and Mississippi Valley floras, and 
would tend to prohibit an interchange of forms except those few 
earth and rock forms able to exist. 
East and south of the Black Hills lies the famous Bad Land 
region of South Dakota and Nebraska. This region is subject to 
hot, dry winds, mainly from the south or southeast, which strike 
the Black Hills on the east and south. As a result tree lichens 
are scarce in this part of the Hills, becoming more and more 
plentiful as we go farther into the heart of the region. The 
larger part of the tree lichens in the collections examined came 
from the cafions and gulches well up in the Hills. Rock lichens, 
particularly species of Placodium, Lecanora and Rinodina, are com- 
mon all along the eastern edge of the Hills, as well as in the foot 
hills bordering them. : é 
Geologically, the Black Hills are largely composed of various 
kinds of sandstone, limestone and granite, all of which furnish | 
