CHICAGO AND VICINITT. 35 



filling processes are carried on is striking ; in pools of known age 

 among the rubbish heaps of Jackson park the author has noticed 

 accumulations of Chara peat amounting to one or two inches per 

 year. 



B. The undrained swamp. — It is obvious that the processes outlined 

 in the preceding paragraph must eventuate in the death of the lake or 

 pond involved and its replacement by a marsh, entirely apart from 

 ordinary erosive activities. Indeed, as has been stated, these activities 

 are relatively unimportant here ; this fact is shown by the absence of 

 -ordinary sediments from most peat beds. As the aquatics make the 

 pond shallower and shallower, they make it more and more unfit for 

 themselves and fit for their successors, viz., those plants which grow 

 along pond margins. Among the first plants of this type are various 

 sedges (Carex), also the bulrush {Scirpus lacustris), though this latter 

 species is more characteristic of the half-drained margins than of those 

 under discussion here. Other marginal plants of our peat bogs are 

 Menyanfhes trifoliata (Buckbean) and Potentilla palustris (Swamp 

 cinquefoil). 



The vegetation that follows may be called typical of peat bogs. 

 The dominant plants are usually shrubs, especially the leather leaf 

 {^Cassandra calyculatd); this plant may be so abundant as to give tone 

 to the landscape. Fig. ig shows some Cassandra islands in a sedge 

 swamp. It is clear that the islands represent places where in the 

 original lake the water was shallow. The present remnant of the lake 

 is shown at the left. Not only have the sedge zones advanced upon it 

 from all sides, but centers of sedge growth appear also in shallow 

 places in the lake itself. Just as the sedge zone encroaches upon the 

 lake, when conditions become favorable, so the Cassandra zone 

 advances on the sedges. Again, a tree zone advances on the shrubs, 

 as will be seen farther on. The zonal arrangement of plant societies 

 that has just been seen is a feature of most peat bogs, and is due to 

 the symmetry of lake and bog conditions. It will be observed that 

 along the lake margin the zones advance toward a common center, 

 while on the islands the advance is from a center. Eventually, of 

 course, the marginal and^ island zones will merge. 



Besides Cassandra many other plants are commonly found in the 

 shrub z(me. Other shrubs are the swamp blueberry {Vaccinium corym- 

 bosuni), the cranberry ( F«m«2«w« macrocarpon), the dwarf birch (Betula 

 pumild), the alder {Alnus incand), the hoary willow {Salix Candida), 



