44 THE PLANT SOCIETIES OF 



blage, like that of the ravine slope, is bound to pass away, though its 

 life tenure is much longer. Sooner or later river action will enter; 

 there will be developed ravines, xerophytic bluffs, and ultinaately flood 

 plains, again with a mesophytic flora. A broad survey then shows a 

 rapid development to a somewhat prolonged temporary climax, and 

 finally, after ravine and bluff vicissitudes, there appears the true and 

 more enduring climax of the mesophytic flood plain. 



3. THE UPLAND SERIES. 



A. The rock hill. — While all of a land area is eventually worked 

 over by stream activities, and can thus be referred to the river series, 

 other activities are at work in a young topography. The swamp 

 series which has just been discussed is one illustration. So also there 

 are hills which are not due to erosive processes, but to other causes, 

 notably, in our region, morainic hills and sand hills. There are rock 

 hills, also, which are not connected with the present erosion cycle. All 

 of these hill types have their peculiar vegetation features, and must be 

 discussed apart from river activities, since they have an interesting his- 

 tory before they are attacked by stream erosion. 



We may speak first of rock hills, which in the vicinity of 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 outcrop 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 mechan- 

 ical erosion, but is much more resistant than most limestones, 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 supposed 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 



