494 MR. G. ERDTMAN: STUDIES IN THE 
2. Alnus. In Finland, Norway, and Sweden the zone where the regular 
appearance of the Picea pollen begins is often used asa valuable standard 
zone (ef. the papers of von Post, Holmsen, Auer 1923, Malmström 1923, 
ete.). This standard zone cannot be employed either along the S.W. coast 
of Sweden, where spontaneous Picea is absent, or in Scotland, where no 
Picea pollen has been found. In Sweden the zone showing the first 
appearance of elm-, lime-, and oak-pollen is sometimes very characteristic, and 
has been used as a standard zone by Sandegren (1916). In Scotland, 
however, the seanty and irregular appearance of the pollen of these three 
species does not permit of such a use. Instead, the zone at, or a little above, 
the first appearance of alder-pollen—the ** A-zone "—seems to be the niveau 
best fitted for the comparison of different deposits. If further investigations 
be carried out, it might become possible to draw maps showing the distribution 
of peat mosses (and the character of the forests) at different periods, The 
pollen-diagrams here published make it probable that the peat deposits 
covered a much smaller area before the A-zone period than they do now. 
In Sjælland and Sweden the first alder-pollen appears in late boreal strata 
(about 6000 years B.C.), and it is probably the same in Scotland. 
When an alder wood grew on the surface of a peat moss, its presence is 
usually indicated by a great amount of alder-pollen in the peat. It is in this 
way that the highest percentages of alder are to be explained (e.g. 58 per 
cent. ; peat moss No. 4). The moors in the neighbourhood of Strathcarron 
(mean percentage —27, maximum=49 per cent.) and Helmsdale show a 
greater alder-pollen frequency than the Achnasheen district and the Isles. 
The Shetland deposits and especially those of Lewis are very poor in alder- 
pollen (mean frequency 6-8 per cent.). 
The pollen of Alnus glutinosa and A. incana is indistinguishable, but in 
Scotland the pollen must belong exclusively to the former species. 
3. Betula. As in Alnus, the pollen of the different species is scarcely 
distinguishable. According to Docturowsky and Kudrjaschow (1923, 
figs. 13-15) there is a difference in the size between Betula humilis, D. alba, 
and D. nana, but from preparations made by myself I find it quite 
impossible to distinguish between them, certainly not between the two latter 
species. 
In no part of Europe which has so far been investigated by the statistical 
method of pollen research, has such an overwhelming dominance of birch- 
pollen been found as in Scotland. Birch was the first tree which invaded 
the country in postglacial time. It is not improbable that in places it was 
accompanied by pine (aspen, rowan, etc., cannot be considered, as the pollen 
of these is not preserved in peat or mud). Along the coast of S.W. Sweden, 
birch-pollen is always dominant in the oldest (pre-boreal) layers. Most 
frequently it is also dominant in late sub-boreal strata and in the subatlantie 
Sphagnum peat, the surface-layer excepted, 
