A LICHEN SOCIETY OF A SANDSTONE RIPRAP. 
BRUCE FINK. 
(WITH FIVE FIGURES) 
Tur ecologic conditions governing the composition of a given 
lichen society are interesting and instructive, though often difficult to 
determine with any degree of certainty. The writer has in various 
papers attempted to trace in a general way some of these conditions, 
treating a considerable number of societies and attempting to show 
how the plants are adapted structurally. Among other societies 
thus studied, there are a number occurring on sandstone, all surrounded 
by very similar climatic but quite different edaphic conditions. Some 
of these societies of the sandstones are surrounded by other lichen 
societies, usually of trees, and show most interesting instances of 
tension lines and invasions of certain lichen species from one to 
another of two adjacent societies. Discussions of these societies may 
be found in the Writer’s papers concerning the lichen floras of Minne- 
Sota and Iowa. 
I. DESCRIPTION OF THE RIPRAP. 
The lichen Society to receive special attention in this paper is 
erg in a number of ways. For some time it has seemed desirable 
na 69 lichen societies of sandstone than those of ledges along 
a “a advantage was taken of the first opportunity for such 
gation far from a large stream by taking a society found 
sowing upon rocks Temoved from their native beds. Before con- 
it will be in order to state its location and to 
the surrounding conditions and antecedents 
existence possible. The riprap on which the 
brace and a protection for a high grade of the 
four miles west of Grinnell, Iowa. The rock 
lower Sic. is constructed is the ferruginous sandstone of the 
Westward on the ous, and was obtained at Kellogg, some thirty miles 
roadbed, and “poe railroad. The riprap lies on the north side of 
1904) € form of a wall along the upper part of the grade 
265 
"ti °ccurs forms a 
ock Island railroad, 
of which th 
