Fhik: ROCK LicirExs of tayi.ors falls. 5 



driven further south than their present position by each of the 

 earlier stages, or ages as the case may be, and retreated with 

 each return of interglacial conditions. The 8 northern species 

 at Taylors Falls are thus either a few of a former flora which 

 has doubtless partly died out and partly migrated northward, or 

 possibly a few species which migrated to the locality from the 

 mountains to the east and w^est during a late glacial stage, as 

 stated above, or even after the final retreat of the ice. Igneous 

 rocks are not supposed to have been exposed over the region 

 covered by the glaciers south of the area now under considera- 

 tion at the time of the first glacial advance, but sandstone no 

 doubt outcropped frequently and probably further south than the 

 glaciers extended. For a long period after each glacial retreat 

 the surface w^as no doubt thickly strewn with rocks left bv the 

 melting ice, and these rocks would furnish abundant substrata 

 for a retreat of the saxicolous lichens to the north. These same 

 boulders, now largely covered, would partly remain at the sur- 

 face durincj interglacial conditions and furnish sufficient foot- 

 hold for the organisms to remigrate during a subsequent ad- 

 vance of the ice, thus taking the place of the sandstone where 

 it was covered by previous drift deposits. Thus several migra- 

 tions, alternately southward and northward, probably followed 

 in succession, and we are studying the last stage in the last 

 northward retreat in this not yet completed series. Of course, 

 it is apparent that the Umbilicaria and many other lichens not 

 now found at Taylors Falls might have flourished on the sand- 

 stone and later on the boulders at a time w^hen the climate was 

 more favorable for northern species than now, at and south of 

 the area under consideration, both as to temperature and mois- 

 ture, and that they could have migrated readily enough with the 

 advances and retreats of glacial conditions. What species of 

 the rock lichens were able to endure these cycles of migration 

 and what were killed out is not easy to conjecture. However, 

 it seems certain that the region was left barren of such life and 

 repopulated several times, and it is extremely probable that 

 enough species survived the migrations, or possibly in part 

 flowed in from the east or west as stated above, to give an arctic 

 or subarctic flora at Taylors Falls for a time after the close of 

 the ice age. 



Since the time when this last northern lichen-flora became es- 

 tablished in the region about Taylors Falls, there has been a 



