380 PAST succession: the ceneosere. 



are found, but which consist throughout their whole mass of well-preserved 

 Sphagnum peat, which rests upon a thin basal layer of aquatic peat. The 

 author investigated a moor of this sort which consisted of an vminterrupted 

 layer of Sphagnum peat from 16 to 18 feet deep. If it is assiuned that this 

 layer of peat has developed without interruption, it seems probable that it 

 could have been formed in 800 or 1,000 years, judging from the present rate 

 of formation of Sphagnum peat. It is very likely that this peat-moor devel- 

 oped in the same period as the others, and that, as in these, the formation of 

 peat was impossible during long periods. However, during the dry periods 

 the greater wetness of the moor prevented the invasion of trees and consequent 

 formation of stump layers. 



Reid (1888, 1898) has proposed the following grouping for the deposits in 

 Britain: 



Neolithic: Post-glacial, but Pre-Roman. "Submerged forests," and alluvial or 

 lacustrine deposits with a temperate flora. Cultivated plants and weeds of 

 cultivation appear. 



Late glacial: Lacustrine deposits of arctic plants, above the latest deposits showing 

 ice action. 



Inter-glacial: Deposits with temperate plants, between strata indicating arctic con- 

 ditions. 



Early glacial: Flood loams with arctic plants, at the base of the glacial deposits of 

 Norfolk. 



Pre-glacial: Newest pliocene deposits (Cromer forest-bed). 



About one-seventh of the British flowering plants have been found in a fossil 

 state and a table is given of the distribution of these in the five periods indi- 

 cated. The following occur in at least four of the periods: Ranunculus 

 aguatilis, R. repens, Caltha palustris, Viola palustris, Stellaria media, Rvbvis 

 idaeus, Poterium officinale, Hippuris vulgaris, Myriophyllum spicaium, Meny- 

 anthes trifoliata, hycopus europaeus, AtripUx patula, Rumex maritimus, Rumex 

 crisjms, Alnus glutinosa, Ceratophyllum demersum, Sparganium ramosum, 

 Potamogetan crispus, Zannichdlia palustris, HeUocharis palustris, Sdrpus 

 paudflmis, Sdrpv^s lacustris, Isoetes lacustris. It is significant that prac- 

 tically aU of these belong to aquatic or swamp associations. 



Sernander (1890) has summarized the Quaternary finds of von Post, Erd- 

 mann, and others, and has re-examined the deposits: 



At a depth of 70 to 80 m., a 0.5 m. layer of gravel covered a 2 m. layer of 

 clay, the bottom of which was 7 m. above sea-level. In the bottom of the 

 clay layer was found Zostera marina together with shells of Mytilus, while in 

 the upper layers Zostera occurred with Popvlus tremvla, Salix aurita, and 

 Eguisetum limosum. In the endeavor to orient the deposits, the author 

 investigated in 1888 a moor known to contain shells of Mytilus. The upper 

 layer of peat contained stiunps of pine, spruce, oak, and alder; this was 

 followed by Phragmites peat, lying upon mud which passed gradually into the 

 clay bed. In the mud were found remains of Mytilus, as well as of Betula 

 verrucosa. It is regarded as probable that the stump layer corresponds to one 

 of Blytt's diy periods, and the upper peat and at least a part of the Phragmites 

 peat to a wet period. 



From the study of various deposits of fresh-water tufa, Nathorst (1891) has 

 reached the following conclusions as to the postglacial cosere in Sweden. The 

 arctic vegetation which colonized the diluvium consisted of Drj/os odopetala, 

 Salix polaris, S. herbacea, S. reticulata, Betula nana, B. intermedia, Oxyria 



