282 BOTANICAL GAZETTE [ocroRer 
are not due in any degree to difference in rock composition, but 
entirely to other ecologic factors. 
C. Some other similar societies. 
The lichen societies of various other sandstones differ somewhat, 
from either of the two considered above. Of those hitherto considered, 
only a single one, that of the Sioux quartzite at Pipestone, Minnesota, 
is isolated in such a way as to show no tension lines or admixture of 
elements that so frequently intrude themselves from other adjacent 
lichen societies. The lichens that have established themselves here 
are a number of Acarosporas, Placodiums, Lecanoras, Rinodinas, 
and Buellias. These lichens in general have strictly crustose thalli, 
well developed and variously chinky, verrucose, and areolate, and 
some of them at least a well developed upper cortex. With these, 
two foliose but closely adnate Parmelias and two similar Physcias 
occur here and there in the society, but do not form a conspicuous 
portion of it. Much of the beautiful wind polishing of the quartaite 
was surely done at latest before the Wisconsin stage of the Pleistocene, 
or shortly after the retreat of the Wisconsin ice, and the writer finds 
the lichens growing on the smoothly polished surfaces, which are ® 
much polished below the lichens as elsewhere. Thus there has been 
no visible change in the surface of the quartzite since the advent of 
the present lichen society, and these lichens with well developed 
thalli have had an abundance of time in which to become establs® 
upon the hard surfaces. There is no doubt but that these species 
may reach an advanced age upon the quartzite, becoming much older 
than is possible upon the more rapidly eroding ferruginous ecsir 
of the riprap, and the finding of all the species in good fruit upon nt 
quartzite is quite conclusive evidence of considerable age. No re 
many of these lichens of the quartzite were growing when gee P 
was built. Yet we find mainly the same species upon the gree 
exposed riprap extension, and this shows that such thalli se iat 
established upon the softer sandstone in a comparatively 
time. ‘ 
2 Fink, B. Contributions to a knowledge of the lichens ta 
Lichens of the Minnesota valley and southwestern Minnesota. Minn. 
2:284. 1899. 
