230 
in places where seepage occurs. No lichens occur, though the rock is more 
stable than usual, with a smaller proportion of shale. 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, Rhus toxicodendron, Vitis and Juniperus 
virginiana, hang from the top of the cliff. This stage continues as long as 
active erosion by the 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 the projecting ledges of limestone. Most of the plants are annuals. The 
following species are typical of such localities: 
Ambrosia artemisivefolia Melilotus alba 
Poa compressa Allium canadense 
Lactuca scariola var. iiitegrata Dipsacus sylvestris 
Nepeta cataria Aster spp. 
Rosa humilis aS 
After direct action by the river has ceased, the talus accumulates 
undisturbed. ‘lhe shale Inyers 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. Xerophytic 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 aryense Melilotus officinalis 
Aster nove-angeliwe Cornus paniculata 
Daucus carota Verbascum thapsus 
Heracleum lanatum Elymus canadensis 
Up to this point, the succession has been almost entirely physiogentic. - 
The plantless stage continues as long as the stream actively erodes the 
