ON THE PLANT REMAINS OF THE SCOTTISH PEAT MOSSES. 821 



5° square. This is the greatest storm frequency of any district at sea-level on the 

 European plateau. According to PL XXIX. of the same Atlas (13), the greatest 

 storm frequency occurs in the months of May, July, August, September, November, and 

 December. This includes just the months when strong salt-laden winds from the ocean 

 would be most fatal to foliage. 



Another factor may be the low summer temperature. The July isotherm of 53*4° F. 

 passes just south of Shetland, turns northward parallel to the coast of Norway, and 

 passes eastward across the White Sea. Shetland thus has a lower mean summer 

 temperature than any other part of Europe at sea-level. During the summer months, 

 north winds are rather frequent, the humidity of the atmosphere is then low, and the 

 temperature frequently sinks below 45° F. 



The strong salt-laden winds with a low summer temperature in my opinion fully 

 explain the absence of woodlands in these islands. This conclusion points to the fact 

 that these climatological features could not have existed during the period when the 

 Lower Forest spread over the islands. 



It will now be well to inquire into the possible causes of the absence of strong salt- 

 laden winds and a low summer temperature during the time of the Lower Forest. 

 This period falls within a time when the land surface in West Europe certainly had 

 a greater extent than it has now. Following on the Mecklenburgian period, marked by 

 the basal arctic bed in Shetland, there occurred a general land elevation in Europe 

 which culminated in the Baltic becoming a fresh- water lake (Ancylus Period of Scandi- 

 navia). To what extent this land elevation spread westward over the British Isles we 

 do not know, but the buried forests frequently found at a considerable depth round the 

 British coasts show that elevation did take place. 



It is a matter for regret that a systematic survey of all these buried forests has not 

 been made in this country. A memoir dealing with the exact depth below O.D. at 

 which such forests occur and their flora would give certain evidence of the amplitude 

 of the changes which took place at this period. All we can say at present is that sub- 

 merged forests occur at depths of 35 feet below mean high- water mark, pointing to a 

 subsequent depression of about 55 feet (14). 



It is generally agreed that the elevation in South Sweden amounted to at least 100 

 feet, and may even have been as great as 260 feet. Even the lesser elevation would 

 considerably increase the land area of Shetland, whilst a rise of 260 feet would connect 

 it with Scotland, and England with the Continent. 



Ustenfeld (15) discusses this question when considering the immigration of the 

 flora of the Fseroes, and comes to the conclusion that a post-glacial land-bridge is 

 necessary to account for the facts. Wakming (15). on the other hand, takes the 

 opposite view. The composition and the relationships of the Fseroese Mora are known 

 in the most detailed manner from the investigations of Danish botanists, and the facts 

 appear to be difficult to explain without a post-glacial land-bridge, particularly as both 

 observers admit that, failing a land connection, wind is the most important agent 



