382 PAST succession: the ceneosere. 



The structure of these deposits is shown in the profiles (figs. 38, 39). At o 

 in figure 38 the area subject to flood passes into a hummocky zone, where high 

 hummocks of turf arise from almost pure slime. Each of these hiunmocks 

 consists within of one or several pine stujnps covered with a light brown peat 

 of Sphagnum palustre, in which are scattered stems of Polytrichum junipennum 

 and leaves of Myrtillm uUginosa. Upon the peat now grows Hyloamium 

 parietinum, which builds a loose decomposed layer of 4 dm. in depth. Upon 

 each hummock grow one or two birches, pines, or alders (c). At b, figure 39, 

 the moor directly touches the water's edge. The soil outside of the moor 

 is covered with stumps, which are rooted in slime about 8 dm. deep and 

 stretch away under the moor. The last is composed of densely packed 

 Sphagnum nemoreum with sparse S. palustre. The present vegetation consists 

 of numerous pines, 9 to 10 m. high, which seem to reproduce readily. In the 

 field layer are to be found the following: Calluna vulgaris, Vaccinium vitis- 

 idaea, Myrtillus nigra, M. uUginosa, Andromeda poUfolia, Ledum palustre, 

 Empetrum nigrum, and Eriophorum vaginatum. The moss layer is formed by 

 Hylocomium parietinum, H. proliferum, and Dicranum undulatum. In figure 38 

 the stump layer lies at a depth of about 1.2 m. With reference to the origin of 

 this stump layer in the peat, Blytt's theory seems to afford the only possible 

 explanation. The insignificant streams which are found in such areas seem 

 quite incapable of reducing the water-content of the peat to any important 

 degree. A long period of dry continental climate, however, could gradually 

 dry out the moor, no matter how wet it were, and thereby further the develop- 

 ment of more and more xerophile forms, until the entire moor becomes covered 

 with scrub or the complete pine forest. This entire development one can see 

 to-day in its various phases over the surface of the moor. After the dry 

 continental period followed a moist insular climate with greater precipitation. 

 As a consequence of the increasing soil acidity, the pines died off and a Sphag- 

 num moor developed. This period is represented by the peat in the profile 

 at b. After this period followed the present with its relatively dry climate, 

 in which the reed-grass swamp gradually changed into Sphagnetum schoeno- 

 lagurosum. This passed into Sphagnetum myrtillosum, which subsequently 

 developed into a pine moor, and then into the usual pine forest. 



On the other hand, Tanfiljew (1891) calls in question the validity of Ser- 

 nander's conclusion, and, on the basis of an examination of similar deposits in 

 Russia, proposes the following explanation: 



When water comes to stand in a forest, the trees will die and the acciunula- 

 tion of tnmks and other material, with the constant addition of water, will 

 cause a rise of level and the flooding of other forest areas. When the new- 

 formed lake reaches a certain level, evaporation and inflow come to equiUbriimi, 

 with the result that fiUing and moor formation occur at the margin. A new 

 forest then develops on the moor, until the level of the lake is again raised by 

 the accumulation of plant remains, the water-level rises, and the trees are again 

 killed. The stinnps are overgrown with moor plants, which form a new layer 

 of peat, and again furnish new soil for trees. If the lake is drained naturally or 

 artificially, or if the destruction of the surrounding forest by fire increases the 

 evaporation too greatly, the conversion into a peat moor is final. 



Blytt (1892), in his investigations of the calcareous tufas of Norway, finds 

 at the bottom a layer of birch tufa, then a layer of mud or hxmius without 

 fossils, and above a layer of pine tufa: 



In the birch tufa, the pine is completely absent. Leaves of Betvla odoraia 

 are very abundant, as well as those of PojyuMs tremula and of Salix. The 

 pine tufa contains throughout an extraordinary number of needles of Pinus 



