44° George E. Nichols, 



period, and may even be present on the higher parts of the 

 mature bog-. The factors concerned may be various, but of 

 particular importance seem to be the depth of the basin to start 

 with and the luxuriance with which the aquatic sphagnums 

 develop. Where these latter are absent or poorly represented, so 

 that the filling in is dependent on encroachment from the margin, 

 the elimination of the pond proceeds slowly. For while the 

 mesophytic, cushion-forming sphagnums may grow luxuriantly, 

 forming great banks of vegetation around the edge of the pond, 

 the centripetal advance into the pond of the fringing banks is 

 usually slow. For this latter fact the commonly sparse develop- 

 ment of the pioneer, skeleton- forming shrubs seems primarily 

 responsible, since wherever an adequate shrubby framework is 

 presented the sphagnums tend to push out from the shore quite 

 rapidly. The banks of sphagnum commonly come to form a 

 complete circle about the pond and block up any natural outlet 

 which may have existed. (Of course, in the case of spring-fed 

 ponds or of any ponds with a considerable outflow, the outlet 

 may not become completely dammed, but such ponds are rarely 

 concerned here in the development of raised bogs.) Thereafter 

 drainage must be accomplished entirely by slow seepage through 

 the peaty banks. As these banks are built up higher through 

 the growth of the sphagnums at the surface, the peat underneath 

 becomes more and more compressed by the superimposed weight 

 and in consequence less and less permeable. The result is 

 obvious : as the drainage becomes impeded below, the surface 

 of the pond is forced to a higher level, and in this way, as fast 

 as the surface of the bog is built upward, the pond likewise is 

 shoved higher and higher, until ultimately it may come to lie at 

 the crest of the mature bog. Concurrently with the changes 

 just outlined, the bottom of the pond may likewise be built up 

 throvigh filling from within, but only when this latter process 

 proceeds at a more rapid rate than that at which the surface 

 of the pond rises can it have any immediate visible efifect. 



Mention has been made earlier of the convex surface which 

 is possessed to a greater or less degree by all raised bogs. This 

 convexity is most pronounced in bogs like the one just described, 

 where there is a central pond which acts as a reservoir and from 

 which water seeps out in all directions. It is self-evident that 

 the areas nearest the pond will be best watered : it is here that 



