1904] FINK—A LICHEN SOCIETY 283 
Neither upon the softer sandstone nor upon the harder quartzite 
has the writer been able to observe any certain evidence of the pro- 
tection which the lichens have afforded the rocks against wind or 
other atmospheric agencies, though other observers find such evidence 
elsewhere on rocks of the same kind.3 But whether the acidic action 
of the lichen thalli upon the rocks, or the climatic, erosion-producing 
agencies acting upon the surrounding rocks causes the more rapid 
disintegration, in the end the two factors together act on the softer 
ferruginous sandstone with comparative rapidity; and as compared 
with the lichen population of the quartzite, that of the sandstone is 
quite transient, lichen thalli or portions of thalli disappearing and 
becoming replaced, except upon the southward exposed extension, 
more rapidly than the better developed thalli can become established 
and produce fruit. So it happens that when lichens having the 
better developed thalli are found, as they rarely are, in the society 
especially considered in this paper, they are likely to be sterile; while 
those with less differentiated and apparently more rapidly develop- 
ing thalli are the ones that are common and well fruited. The fruti- 
cose species, as the Cladonias and the Stereocaulon, are rather rarely 
established upon the firmer and more exposed rocks. In their more ~ 
or less shaded and moist habitat in the holes in the riprap, or in pro- 
vanes Places about the basal blocks, these fruticose species are able 
0 maintain themselves in spite of disintegration, the wind not blow- 
ing them away as is the fate of the smaller thalli on the more exposed 
surfaces, aS soon as these thalli and the atmospheric agencies together 
disintegrate the rocks sufficiently. Finally, any of the fruticose forms 
that attempt to gain a foothold on the more exposed surfaces are 
Probably even more likely to be blown away as disintegration pro- 
ae than are the crustose forms, though the rhizoids of the former 
he Tate the rocks to greater depth than do the hyphal rhizoids of 
latter, 
ris _ Societies of the Saint Peter sandstone along the Mis- 
been consid er 034 Minneapolis and south of McGregor, Iowa, have 
ae ered in a previous paper‘, and are quite different from the 
discussed chiefly in this paper; and the same may be said of the 
* SHINER, B, Livi 
* Fink, B 
ng plants as geologic factors. Proc. Iowa Acad. Sci. 10 : 42. 1902. 
+» Notes ©oncerning Iowa lichens. Proc. Iowa Acad. Sci. 5 : 180. 1897: 
