Bergman and Slallard: climax formations 347 



where the surface is of sohd rock, representing an extreme de- 

 ficiency in water-content. The most extensive area of this kind is 

 in northeastern Minnesota. Smaller areas occur along the south 

 side of Lake of the Woods. These rocks are probably of igneous 

 origin and belong to the Archean period. They consist of green- 

 stones, quartzites, granites, gneisses, schists, and jasper variously 

 intermingled. In northern and northeastern Minnesota, these Arch- 

 ean rocks have remained at the surface since the Archean period, 

 while in other parts of the state except the southeastern the rocks of 

 that period have been covered by sedimentary formations of subse- 

 quent periods. The passage of the glaciers during the Glacial period 

 planed down the Archean rocks somewhat and left deposits of 

 glacial till over some of them, but extensive areas still remain ex- 

 posed. These constitute the bare areas for the initiation of xerarch 

 succession. 



Weathering of the exposed rock surface has contributed to the 

 development of vegetation within that area. In some places the 

 rocks are still as smooth as they were left after being worn down 

 and polished by glacial action, and on these little or no vegetation 

 is to be found. On others the surface of the rock has been disin- 

 tegrated by weathering, in places to a depth of several inches, and 

 in such places a dense covering of vegetation has developed. 



A. succession having its origin on rock and culminating in the 

 Pimis climax association also shows a series of progressive stages. 

 The stages of xerarch succession, however, are not as readily traced 

 as those of the hydrarch and complete data are not yet at hand. 

 From the available data of quadrats, transects, and field notes, the 

 stages seem to be as follows : 



(1) Crustose Lichen associes 



(2) Cladonia-P olytrichum associes 



(3) Herbaceous associes 



(4) Jiiniperus associes 



(5) Pinus-Betula associes 



(6) Piiius association 



(1) Crustose Lichen Associes 



On bare hard rock owing to the extreme deficiency of water and 

 the exposure to which they are subjected, crustose lichens alone are 

 able to gain a foothold. Since these are able to grow during periods 

 of wet weather and to remain in a state of dessication for an in- 



