432 George E. Nichols, 



portions of the hog push out into the pond, these lower areas 

 commonly become completely engulfed. The vegetation of the 

 hollows thus formed is strikingly different from that in the sur- 

 rounding bog, being essentially similar to that of the muck mat 

 described in earlier paragraphs. In addition to the two sedges, 

 Carex filiformis and Rynchospora, the following vascular plants 

 are characteristic : Schisaea pnsilla, Lycopodium inundatum, 

 Carex oligospcrma, Ranunculus Flammula reptans, Drosera 

 longifolia, and Utricularia cornuta. Ultimately these depressions 

 seem destined to become incorporated with the rest of the bog, 

 but they may persist virtually unaltered for a long time. The 

 usual forerunner of the typical bog vegetation is Scirpus 

 caespitosns, and this sedge seems to pave the way for the 

 tushion- forming sphagnums which ultimately gain control. 



It has already been noted that in the small ponds of the bar- 

 rens, where the aquatic sphagnums play such an important part 

 in the filling process, the mucky substratum which these form, 

 with its felty cover of Sphagnum Pylaisei, S. cuspidatum, and 

 Cephalozia, may similarly persist virtually without further 

 change for a very long time. It may be added here that very 

 commonly such areas are gradually being reduced in size and 

 seem destined to extinction through the slow centripetal encroach- 

 ment of the steep, peripheral banks of sphagnum. 



The method of filling which has been described in the pre- 

 ceding paragraphs differs from that previously described as due 

 entirely to "filling from within" mainly in the more obvious 

 centripetal encroachment of the marginal swamp vegetation. As 

 a matter of fact, there is scarcely any real distinction, for, 

 strictly speaking, as soon as the bottom of a pond has been 

 built to the surface through the activity of the aquatic plants, 

 any further changes are invariably due to the invasion of plants 

 from without. 



The climax association-type of bogs. — Extended comment is 

 hardly necessary. The character of the climax association-type 

 varies locally, but in general it is marked by the predominance 

 of sphagnums and ericaceous shrubs, with black spruce and 

 tamarack. In many cases the vegetation of ordinary bogs in 

 the highland is scarcely different from that described for low- 

 land bogs, while in others it closely approximates the conditions 

 found in raised bosfs which will be described next. 



