362 BOTANICAL GAZETTE [NOVEMBER 
(Castor canadensis Kuhl) has been an important factor in the creation 
of bog areas (37), and in the extension of areas already existing, by 
the building of dams. The beaver was found in this section when it 
was first settled, but the last known specimen was killed sixty-nine 
years ago. The occurrence of peat deposits several feet in thickness 
and covering quite large areas, bordering streams, whose channels 
lie deeply sunken in the deposits, seems to find its best explanation 
in this manner. But little field work has been done on the relation 
of beavers to the peat deposits, and examples are still too hypothetical 
to cite in this connection. 
BOG AND LAKE VEGETATION. 
Of the plants which might come into a new land area containing 
basins, such as was laid bare on the retreat of the glaciers, none is 
better adapted to rapid migration than the group of aquatic plants. 
Whether. we have in mind the smaller submerged varieties or the 
partially submerged littoral species, their wide geographic distri- 
bution and uniform associations bespeak their evident solution of the 
problems of dispersal. The fact that deposits of peat and marl have 
been found in northern Indiana and lower Michigan to a thickness 
of 40 feet (12™) would indicate that in these particular basins the 
vegetation must have obtained an early foothold. 
Concerning the deposition of marl, it is of interest to us only in so 
far as it becomes an agent of aggradation in the basins. an the 
reports (5, 42, 21) on the marl deposits of Indiana and Michigan, 
many examples are cited where the marl forms the underlying sub- 
stratum of peat deposits. That its deposition to a large extent IS due 
to plant life has been shown by Davis (9, 10). The plants most 
concerned with this process are the Characeae and Cyanophyceae 
(Schizothrix, Zonotrichia). They are probably aided : i 
‘ 7. ar ti : s or 
mollusks, and perhaps also by chemical precipitation eee 
Characeae and Cyanophyceae, they have a wide range us 
different lakes, and may occur in deep or shallow water an - 
various rock substrata. Where they come into competition i 
shore species, the rank growth of the latter usually precludes ee 
existence in sufficient amount to be of importance in marl ane 
Where wave action is strong, the chara is confined to deeper neces 
