348 George E. Nichols, 



much so, in fact, that the shoreward parts of a lake may have become 

 completely filled in before any appreciable accumulation has taken place 

 in the deeper areas. The filling in of deep lakes usually proceeds centri- 

 petally. This is due to the fact that the shoreward zones of vegetation, in 

 consequence of their more vigorous growth, exhibit a tendency to push 

 outward into deeper water. Where this tendency is pronounced, the shoal 

 water zones may completely override the deeper water zones, at the same 

 time causing the lakeward slope of the deposit to become much steeper. 

 The filling in of the deeper parts of a lake may also be effected to a 

 varying degree by the accumulation of loose debris from the adjoining 

 shallows or by the deposition of sediment in flood time, while various 

 plankton forms may contribute in a small measure to the deposit. 



Coincident with the upbuilding of the substratum through the deposition 

 of muck or peat, as outlined in the preceding paragraph, transformations 

 occur in the character of the vegetation growing on the lake's bottom. 

 For, as the depth of the water diminishes, it becomes possible for plants 

 to develop which were unable to grow in the deeper water. And as 

 these shallow water plants increase in number and abundance, they may 

 crowd out and eventually replace the deeper water species. Thus there 

 may follow one another a series of plant associations, each one of which, 

 by helping to raise the bottom of the lake to a higher level, prepares the 

 way for less hydrophytic associations, but at the same time, by so doing, 

 brings about its own extermination. 



It is a familiar fact that the plants which fringe the edges of so 

 many lakes are commonly massed in more or less definite bands or zones 

 that tend to be concentric with respect to the deeper parts of the lake. 

 The floristic composition of these zones in any given lake is determined 

 largely by the ecological requirements of the various species of plants 

 which happen to be present, in relation to the depth and clearness of 

 the water . . . Reference has already been made to the succession of 

 plant associations which accompanies the building up of the lake bottom. 

 It has been found that this dynamic Vertical Succession corresponds 

 closely with the apparently static Horizontal Zonation just outlined 

 This general coordination between the contemporaneous hori- 

 zontal sequence of zones and the historical or vertical order of suc- 

 cession has been verified repeatedly by the stratification of plant remains 

 observed in peat deposits, and is of great assistance in reconstructing the 

 past or predicting the future course of events in any specific locality. 



Of course, not all swamps have originated in the manner just 

 described (see further under head of swamps) ; neither, on the 

 other hand, do all lakes exhibit any pronounced tendency to 

 become converted into swamps. For reasons which are not 

 always clear, there is the greatest variation in the speed at which 

 the transformation is brought about, and in many lakes, not 

 only does there seem to be scarcely any tendency toward swamp 



