MICROPALÆONTOLOGY OF POSTGLACIAL DEPOSITS. 493 
a quantitative method for microscopical analysis. But the methods involve 
great expenditure of time, and even then the resulting numbers are not quite 
comparable, because of the varying rate at which peat forms. 
As to the Faroes, it must be remarked that the above-mentioned PF figure 
is based upon a single peat moss only, from which 7 specimens were 
collected for microscopical analysis. 
It has been mentioned that in the upper layers of the peat deposits, 
especially those of the Shetlands, Orkneys, and Lewis, tree pollen is 
practically absent. When the pollen-flora of Scottish deposits has been more 
thoroughly explored, it would be a fascinating occupation to compare these 
changes in the pollen frequency with the state of woodlands in old times, as 
described by historiographers such as Herodian, Solinus, Tacitus, Torfæus, 
and others. 
Where absolute frequency figures are lacking, relative ones have often 
been used. For instance, it has been stated by several authors (e.g. Sandegren, 
1916, and von Post, 1916) that the PF in most cases is greater in highly- 
decayed than in slightly-dec: ayed peat. To express the degree of humification, 
von Post used a scale of 10 divisions, 10 being the highest (ef. p. 4). 
Sandegren (l.c.), using von Post’s scale, obtained :—from a layer of pre- 
subatlantic Sphagnum-peat with humification represented by 9, a PF=345; 
from layers with humification 8 and 7, respectively, PF’s=25 and 17 ; from 
subatlantie peat with humification 3 or 2, as well as from slightly-decayed 
pre-subatlantic peat, PF=17°5 (these figures are calculated upon those of 
Sandegren, who expressed the PF as the mean number of pollen-grains per 
preparation, 18 x 18 mm. ; the relatively low figures indicate that he used 
preparations much thinner than those used by the author). 
Other suggestions as to the PF might be obtained from Halden, 1922 and 
Auer, 1923. The former (L c. fig. 5, p. 21) figures a PF curve from a 
deposit with a postglacial marine-clay bed situated between two peat strata. 
When compared with other curves, this PF curve serves to show the layer of 
the marine clay which must have been formed when the land-surface was at 
its lowest level. The latter (l. e. fig. 66, p. 218) gives a diagram illustrating 
the PF for different kinds of peat in different stages of decay. 
4. Tur Forest Trees. A Summary of the Occurrence of their Pollen-grains 
in the Deposits investigated, and a Comparison with the Occurrence of 
Pollen in the Peat Mosses of Sjælland * and S. W. Sweden * 
1. Acer. Maple-pollen is exceedingly rare, only haif-a-dozen pollen-grains 
having been counted from the Shetlands, Orkneys, Hebrides, and Strath- 
carron (Ross). Its behaviour is exactly the same in Sjælland (Jessen, l. c. 
p. 187, and S.W. Sweden (Erdtman, l. e.; Halden, 1922, p. 13). 
* According to Jessen, 1920, and Erdtman, 1921. 
