Ixxxvili FLORA ORCADENSIS. 
is the absence of certain animals that are common in 
Sutherland and Caithness. Thus the frog and the 
adder do not occur in Orkney, though the toad is 
found there in great abundance. It is difficult to 
explain this except on the hypothesis that the land 
connexion did not last a very long time; at anyrate, 
this argument has been advanced to explain the 
absence of snakes from Ireland. 
Perhaps the Pentland Firth did not exist in its 
present form at the time when the ice melted away. 
It may have been a narrow valley which had been 
cut by some pre-glacial stream. At first the North 
Sea was full of ice, and the water that arose from the 
melting of the snow found an easy escape through the 
Pentland Firth at a time when the North Isles were 
still covered by ice of considerable thickness, which 
blocked the outlet in that quarter. In this way the 
Pentland Firth would be soon widened and deepened 
so as to become a formidable barrier to the passage of 
land animals northward from Caithness. Hence the 
meagre fauna of Orkney may represent only the first 
animals that succeeded in crossing before the way 
was barred. It is supposed that the English Channel 
took its origin in a similar manner. 
PEAT DEPOSITS. 
The fullest record of the history ot the Orkneys 
in post-glacial tinies is contained in the peat deposits 
that cover so large a part of the islands. Except 
where it has been cut for burning or removed by 
cultivation, peat extends over the most of the interior. 
It may be roughly classified into hill-peat and bog- 
