Igor | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 149 
Many bogs of this type are very spongy and unstable, whence 
the name quaking bogs; this feature is due to the rapid growth 
of the vegetation and the absence of ordinary inorganic soils for 
a considerable depth. The similarity of the peat bog vegetation 
throughout the northern hemisphere is one of its most striking 
features. Not only the adaptations but the species themselves 



- 20.—Tamarack swamp in an undrained portion of the Calumet flood plain 
at ‘fie. Peat bog herbs and shrubs in the foreground. 
are similar over vast areas; the conditions are unique and the 
flora also. None of our plant societies, not even the lakeward 
dune slopes, have sucha pronounced northern flora as do the peat 
bogs. No contrast could be more striking than that between 
the southern vegetation of the flood plains and the northern 
flora of the bogs. 
fig. 19 shows that a coniferous vegetation, now represented 
by but two or three small trees at the centers of the islands, is 
to follow the Cassandra. Such an advance of conifers on 
Cassandra is shown in the background at the right. The most 
typical conifer in such cases is the tamarack (Larix Americana) ; 
With this the arbor vitae ( Zhaya occidentalis) is sometimes found. 
