400 PAST succession: the ceneosere. 



in complete agreement with Blytt and Sernander as to the fact that those beds 

 occupy definite horizons in the peat. The view brought forward by Gunnar 

 Andersson in 1909, that 'stool occurrences (tree roots) in the peat mosses of 

 Scandinavia represent all the phases of the Post-glacial Period, and that even 

 if they are localized in one peat moss to a certain horizon of the peat, one has 

 no right, at least on the strength of investigations undertaken hitherto, to 

 parallelise them with another as to time,' receives no support from the British 

 peat areas, and in Iceland a continuous layer of trees is found in districts which 

 now lie beyond tree limits." 



Recently, Fries (1913) and Wille (1915) have given detailed accoimts of the 

 immigrations and shiftings of the Scandinavian flora. Much of this is of great 

 interest in connection with the peat costase, and some of it bears directly upon 

 the succession of peat horizons, but the limitations of time and space have 

 made an adequate consideration of them impossible. 



Weed (1889) found that diatom beds of recent origin cover many square 

 miles in the vicinity of the geysers and hot-spring basins of Yellowstone Park: 



These deposits are still forming in the warm marshes supplied by the hot- 

 spring waters. The diatom ooze seems to be apparently bottomless, and the 

 marshes are correspondingly treacherous. A typical marsh of this character 

 is found near Emerald Springs in the Upper Geyser Basin. In times past the 

 water has encroached upon the adjacent forest of Pinus murrayana and the 

 bare trunks of the latter still stand upright in the ooze, or lie scattered or half- 

 immerged in the water. A subsequent partial recession of the water has left 

 a bare white strip between the bog and its original margin. This area has a 

 feeble vegetation growing on white, powdery, diatomaceous soil. A large 

 part of the bog is now covered with a sparse growth of water-plants, while the 

 drier parts are grass-grown and form a fairly firm meadow. The greater 

 portion, however, still consists of a semi-liquid, greenish-gray ooze composed 

 of the following diatom genera: Denticula, Navicula, Epithemia, Cocconema, 

 Fragillaria, etc. It has also been found that the meadows of the geyser and 

 hot-spring basin were once marshes of the same character, since they are 

 underlaid by beds of straw-colored or gray material consisting of diatoms. 

 These diatom beds cover many square miles in the vicinity of active or extinct 

 hot-spring vents and are from 3 to 6 feet thick. The wagon-road to the 

 geyser basins crosses a meadow of this character just south of the Norris 

 Basin, and the meadows of the upper and lower basins of the Fire-Hole River 

 are of a similar nature, as shown by the square blocks of dried diatom earth 

 along the roadside. In most of the cases observed, diatom marshes cover 

 ancient deposits of siliceous sinter. 



Penhallow (1900 : 334) has made by far the most important contribution to 

 the study of the Pleistocene clisere of America. This has already been referred 

 to, but it seems desirable to give a more detailed account of his results here: 



"Only one species appears to have disappeared in Pleistocene time. Acer 

 pleistocenicum, which was abundant in the region of the Don, bears no well- 

 defined resemblance to existing species. With this one exception, it is a 

 noteworthy fact that all the plants of the Pleistocene flora were such as are 

 now represented in the same locaUties, or, in the case of the Don Valley, by 

 plants which find the northern limits of their distribution at or near that region, 

 and the somewhat unequal distribution thus indicated at once suggests defimte 

 climatic changes during Pleistocene time, as represented by the northern and 

 southern migration of particular types of plants. This has already been 



