362 BOTANICAL GAZETTE [NOVEMBER 



(Castor canadensisKuhl) 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 (i2 m ) 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. In 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 by certain 

 mollusks, and perhaps also by chemical precipitation. As for the 

 Characeae and Cyanophyceae, they have a wide range of habitat in 

 different lakes, and may occur in deep or shallow water and on 

 various rock substrata. Where they come into competition with 

 shore species, the rank growth of the latter usually precludes their 

 existence in sufficient amount to be of importance in marl formation. 

 Where wave action is strong, the chara is confined to deeper water, 



