CHICAGO AND VIC IN ITT. 



45 



plants is an abundance of Solidago nemoralis. Other species growing 

 in the crevices or on the first soil which is formed on the rock face are 

 Potentilla arguta, Verbascum Thapsus (Mullein), Heuchera hispida (Alum 

 root), Poa compressa (Wire grass), etc. At Thornton there is a rock 



Fig. 26. — Slope of limestone ledge at Stony Island, showing mosses and higher plants 

 establishing themselves in the crevices. 



outcrop which gradually recedes from the surface, and it is possible to 

 tell by the vegetation where the rock surface dips considerably under 

 the surface of the soil. Where the soil is shallow the dominant plant 

 is Poa compressa, but as the soil layer deepens it becomes gradually 

 replaced by Poa pratensis (Blue grass). Similarly, at Stony -Island, 

 crevices can be distinguished in a covered horizontal rock surface by a 

 sudden change from the xerophytic plants of the shallow soil, that hides. 



