158 BOTANICAL GAZETTE [MARCH 
Chicago are quite rare and consist entirely of dolomitic Niagara 
(Silurian) limestone. Not only are hills of this limestone quite 
rare, but surface outcrops of any kind are uncommon because of 
the heavy drift. Hence the rock vegetation of the Chicago 
area is not very important. Perhaps the most interesting out- 
crop is at Stony island, where it is quite easy to trace the 
various stages in the development of the vegetation. This 
rock, like most limestone, is subject to chemical as well as 
mechanical erosion, but is much more resistant than most lime- 
stones on account of its strongly dolomitic character. The first 
vegetation that gets a foothold is composed of lichens, but the 
lichen flora appears to be rather sparse, perhaps because of the 
chemical nature of the rock, since lichens are commonly sup- 
posed to shun calcareous soils. The relative poverty of lichens 
may be due, however, to the easy solution of the surface rock 
layers and the consequent difficulty in retaining a foothold. 
The limestone is considerably jointed and fractured and there is 
in consequence a rich crevice vegetation, composed of several 
mosses, especially Ceratodon and Bryum, and also various 
grasses. Fig. 26 shows a vegetation of this nature, and among 
the other crevice 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 T; hap- 
sus, Heuchera hispida, Poa compressa, etc. At Thornton there is 
a rock 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. 
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 most of the 
rock, to the mesophytic plants of the deeper soil which lies 
over the crevices. 
’ Through rock decay and the accumulation of organic matter 
a considerable soil comes to be developed where there was at 




