Ixxxvi FLORA ORCADENSIS. 
mergence of which the buried forests give evidence 
may amount to seventy feet or more. Dr Peach 
informs me that he was told by the late Mr Traill 
that great blocks of peat are torn up by tides and 
storms from the bottom of the North Ronaldshay 
Firth, and that in some of the Shetland voes the 
fishermen bring up peat on the flukes of their anchors 
where the water is by no means shallow. 
If we look at the charts of the North Sea, 
we find that an elevation of about 200 feet would 
connect Caithness with Orkney and convert part 
of the Pentland Firth into dry land. No buried 
forests have been found at so great a depth in the 
North Sea. If Orkney has ever been joined to 
the mainland of Scotland during post-glacial times, 
an elevation of nearly this amount must have 
taken place. Of course it might be argued that 
the Pentland Firth has been deepened by the 
scour of the strong tides that rush through it, but 
this action, which has undoubtedly gone on, would be 
confined to the narrow part of the strait, and there 
might have been a heaping up of sandbanks where 
the current slackens. Accordingly, this estimate gives 
us a fairly exact idea of the amount of elevation that 
would be necessary to join Orkney to the mainland 
of Scotland. 
There are certain strong reasons that would 
lead us to believe that the strait had been inter- 
rupted by a land connection at least once since the 
ice that deposited the boulder-clay melted off the 
face of the Orkneys. Many wild animals are to be 
found in the islands, principally rodents, such as 
