MICROPALÆONTOLOGY OF POSTGLACIAL DEPOSITS. 469 
D. 10 em. do., with numerous Empetrum stems. 
E. 25 em. do., with numerous birch branches and twigs of at least as 
large a size as those found in the upper forest bed, 
F. Moraine. 
From this section Samuelsson concludes that the district has twice been 
covered by a birch forest, which has been replaeed by more hydrophilous 
associations, and he suggests that the forest-remains found by Lewis belong 
to the Upper Forest. That Lewis nowhere found any analogy to the Lower 
Forest may, in Samuelsson’s opinion, possibly depénd upon the character of 
the oldest strata (lake deposits) at the point where he found the most 
complete stratification, 
Upon the island of Lewis I have studied (in sections and with the bore) 
20 peat deposits of different kinds, ranging in depth from 100 to 520 cm. 
The combined thickness of all the strata investigated may be about 40 m. 
In these deposits tree-remains have been met with but seldom, and in most 
cases the deposits seem to have originated in boggy ground. The lack of 
more detailed investigations makes it impossible to parallel, as Samuelsson 
does, the two birch layers with the Upper and Lower Forest beds 
respectively ; and it seems equally impossible to state whether the birch 
stratum described by Lewis corresponds with the upper or with the lower 
layer found by Samuelsson. The statistical method of pollen research often 
shows the untenableness of such over-hasty parallelisms, the pollen-curves in 
a forest-bed of a certain moss being almost the same as those (e.g. in a layer 
of Sphagnum or Amblystegium peat) in another, although in the latter there 
are perhaps two forest-beds lying, the one above, and the other below the 
more hydrophilous stratum. 
The stratification of peat or mud (gyttja) deposits often furnishes the most 
decisive proof as to changes in postglacial climate. From the deposits of 
Middle Europe, Gams and Nordhagen (1923) have furnished a valuable 
contribution to the criteria for estimating such changes. In some countries, 
however, either peculiar topography (Finland: cf. Auer, 1923) or the 
influence of unremitting climatic factors (Scotland, or at least parts of it) 
may have been of such importance that local edaphic factors have given 
their stamp to the deposits, or have to a certain extent prevented them from 
showing those characters which would have been theirs if the general 
climatic conditions had been able to exercise an undisturbed influence upon 
their development. The importance of edaphic factors has been accentuated 
by many authors ; most recently by Salisbury (1921 & 22). On the other 
hand, Gams and Nordhagen (/. e.) consider that there have been further 
climatic changes (e. g. during the subatlantic time there must have been at 
least one drier period), and Gerassimoff (1923) suggests that the alternation 
of many thin pine-trunk layers with strips of slightly decayed Sphagnum, 
as described by him from the peat moss of Galitz (Tver), may be due to slight 
changes of climate. 
