113 



more diflFerent than the two iust described. The differences become more 

 striking when it is considered that the two successions are both on lime- 

 stone, in the same region and on cliffs extending in the same general 

 direction. The principal differences are as follows: (1) The succession 

 on Hudson River limestone is more rapid than that on Niagara lime- 

 stone. (2) There is a striking contrast in the pioneer stages. The 

 pioneer association on Hudson River limestone is characterized by the 

 complete absence of lichens, liverworts, xerophytic mosses and ferns, all 

 of which are prominent on Niagara limestone. (3) In the Whitewater 

 Gorge, the degradation of the cliffs of Hudson River limestone is accom- 

 plished by the crumbling of the rock into small fragments, while at Elk- 

 horn Falls the fragments of Niagara limestone are of many tons' weight. 

 (4) On account of the ovei'hanging character of the cliff at Elkhorn 

 Falls, an intermediate mesophytic stage is introduced into the succession. 



8—11994 



