2H0 



In places where seepage occurs. No liclicns occur, tlaough the rock is more 

 stable than usual, with a smaller proportion of sliale. Their absence is 

 not due to smoke, as is sometimes the case, for they occur on trees near 

 by. The rock does not contain bitumen -which sometimes prevents their 

 growth, notably on Niagara limestone. It seems probable that the weath- 

 ering of the rock is too rapid for them to maintain a foothold. A few 

 annuals grow on the talus which has accumulated since the spring floods. 

 A few plants, such as Psedera, Ithus toxicodendron, Vitis and Juniperus 

 virginiana, hang from the top of the cliff. This stage continues as long as 

 active erosion by tlie stream is maintained. 



The second stage is found at a point where the river erosion is not 

 so strong. A considerable talus accumulates at the base of the cliff, and 

 this is not swept away by the spring floods. The wall is not so steep as 

 in the stage just described. It is in this stage that the first real plant 

 associations appear. These pioneer plants occupy narrow shelves produced 

 by tbe projecting ledges of limestone. Most of the plants are annuals. The 

 following species are typical of such localities : 



Ambrosia artemisia'folia Melilotus alba 



Poa compressa Allium canadense 



Lactuca scariola var. iutegrata Dipsacus sylvestris 



Nepeta cataria Aster spp. 

 Rosa humilis 



After direct action by the river has ceased, the talus accumulates 

 undisturbed. The shale layers change to soil very readily, and this is 

 washed down by the rains. Projecting layers of limestone break off of 

 their own weight. In these various ways, the slope is rapidly reduced. A 

 larger number of plants gain a foothold and the cliff" is covered with vege- 

 tation. Grasses and annuals are common. Xei'ophytic mosses appear. 

 This may be called the herb stage. The pioneer plants mentioned above 

 continue through this stage, while the following new species appear : 

 Equisetum arvense Melilotus oflicinalis 



Aster novse-angelite Cornus paniculata 



Daucus carota Verbascum thapsus 



Heracleum lanatum Elymus canadensis 



Up to this point, the succession has been almost entirely physiogenic. 

 The plantless stage continues as long as the stream actively erodes the 



