Fink: ROCK LICHENS OF TAYLORS FALLS. 7 



years ago. Thus it seems that the succeeding 8,000 years must 

 have sufficed for the establishment of a more or less rich Arctic 

 flora and the gradual change to present floral conditions. The 

 relative times involved in the establishment of the first flora and 

 the gradual change to the present cannot be arrived at, since the 

 richness of the first cannot be known, and we cannot yet be sure 

 that a portion of the species migrating southward were not 

 killed out in some portions of the series of migrations, so that 

 some portion of the northern species that became established in 

 the locality would have to migrate toward the center of the con- 

 tinent from the southward-extending mountains already men- 

 tioned. Light on this last supposition, which can only be fully 

 obtained, it seems to me at present, by a study of the lichen flora 

 of the British possessions far to the north of Minnesota, would 

 be extremely interesting. 



The absence of the 8 northern lichens from the sandstone 

 may be easily explained, since it seems that the present sand- 

 stone surfaces exposed between Lake Superior and Taylors 

 Falls are largely or entirely due to post-glacial erosion. If 

 some of these surfaces are admitted to be as old as the time of 

 the last glacial retreat, doubtless Arctic species grew on them at 

 some time subsequent to that retreat. If this be true, it is yet 

 easy to account for the failure of these lichens to persist on the 

 sandstones as well as on the igneous rocks, since the lichen-flora 

 of these porous and easily eroding surfaces must be a compara- 

 tively changeable and transient one, so that whatever such 

 species once inhabited them would now be replaced by species 

 more characteristic of present climatic conditions. After the 

 final retreat of the ice and the change to present conditions 

 of temperature and moisture began, the rapidly eroding surfaces 

 would begin to lose their northern species and be resupplied by 

 those at hand on other substrata at once, while those on igneous 

 rocks could be replaced, mainly, at least, only by a fierce and 

 long-continued struggle between the Arctic and temperate floral 

 elements. The large number of species found on the sand- 

 stones is at first surprising, for while the igneous rocks are much 

 richer in individual lichens, they show no appreciable advantage 

 in species. The softer texture of the sandstone, which caused 

 the more rapid destruction of the species growing under un- 

 favorable climatic conditions, has also brought this condition. 

 To be a little more explicit, while on account of their rapid ero- 



