238 



All stages from young ravines, with conditions extremely xerophytic up to 

 matui-e ravines with mesophytic vegetation are found. 



The first stage of one of these ravines is merely a shallow groove down 

 the side of the bluff, floored by the bare bedrock and partially choked by 

 rock fragments. The debris accumulates as a coue at the foot. In this 

 stage, a ravine is extremely xerophytic. Few if any plants grow within 

 them, though a few appear upon the talus at the foot. The plants which do 

 appear are the same as the pioneer plants on a cliff face. 



The position of such a ravine seems to be determined almost entirely 

 by the surface drainage outside the gorge. Wherever the topography 

 of the surface outside the gorge causes the flood water to be dis- 

 charged down the bluff, a ravine will be formed. Cleavage planes, which 

 commonly determine the location of ravines in more massive rock, are 

 absent. Seepage lines which often determine ravines in clay bluffs prob- 

 ably have little effect here, for they often occur on cliff faces where no 

 tendency to ravine formation is evident. The rapidity with which a ravine 

 will grow is of course dependent upon the water supply. 



Older ravines are generally not regular in gradient, but become very 

 precipitous in some parts, on account of the occurrence of occasional harder 

 layers of limestone. This produces vertical faces from a few inches to six 

 or eight feet in height, and these are usually wet with seepage or run-off 

 trom above. These ravines are usually quite deep and well shaded. Ver- 

 tical faces of the kind described are commonly covered with Cladophora 

 and Yaucheria. Where the water is contaminated by sewage. Oscillatoria 

 replaces these. Mosses grow luxuriantly in these situations, but no liver- 

 worts grow anywhere in these ravines, with the single exception of Porella. 

 which is common in mesophytic situations throughout the region. The 

 absence of liverworts is difficult to account for, in view of the hydro- 

 mesophytic conditions which prevail in such situations. Fimbriaria and 

 Fegatella were found in abundance on damp rock shelves near Thistle- 

 thuaite's Falls, north of Richmond. Aneura and Blasia were found on 

 clay in the same region. Marchantia occurs on rocks in similar localities 

 near this point. 



Why mosses should be so abundant and liverworts entirely absent in 

 these ravines is difficult to explain. The finding of several genera of liver- 

 worts on similar rock shows that their absence is not due to the chemical 

 nature of the rock. The older parts of the Aneura found on clay were stiff 



