"POSTGLACIAL FORMATIONS" OF SCOTLAND 677 



and the larger rivers were then chiefly engaged in filKng up the 

 lower reaches of their valleys with the abundant sediment brought 

 to them by their active tributaries. In a word, epochs of dominant 

 erosion alternated with epochs of prevalent deposition. 



I might also cite, in support of my general conclusions, certain 

 facts relating to the present and past geographical distribution of 

 animals and plants. The appearance, for example, in the North 

 Atlantic, of isolated colonies of southern types of molluscs, surrounded 

 on all sides by boreal and cold- temperate forms; and the occurrence 

 now and again of similar no longer indigenous molluscs in the raised 

 beaches of Nova Scotia, Greenland, Spitzbergen, and Scandinavia, 

 are all ahke strongly suggestive of warmer conditions having at 

 a very recent period characterized the North Atlantic. Quite in 

 keeping with these phenomena is the fact that the beaches in question 

 are often crowded with southern types which, although still lingering 

 on in these northern seas, do not now attain so large a size as their 

 postglacial predecessors, while they are obviously much less abundant. 



But these, and other lines of evidence, suggested especially by 

 the geographical distribution of plants in temperate Europe, cannot 

 be considered at present. 



Although the proofs of alternating genial and ungenial climates 

 supplied by our peat-mosses seem to me too strong to be resisted — 

 fortified as they are by the evidence yielded by our raised beaches 

 and recent morainic accumulations — I have yet long felt that they 

 would probably be still further confirmed if our peat-mosses were 

 subjected to a thorough examination by competent botanists. I 

 could not doubt that a careful study of the constituents of our peat- 

 mosses would throw light on the changing character of the climate 

 during the period of their accumulation. It was obvious, even to 

 me who am no expert in botany, that peat was composed of other 

 plants than the bog- moss — in many sections I could see what appeared 

 to be a succession of layers made up of the remains of different 

 kinds of plants. And often have I regretted the botanical ignorance 

 which forbade any attempt on my part to interpret what that succes- 

 sion meant — that it had some interesting tale to tell I did not doubt. 

 Fortunately the work of interpretation has at last been taken up by 

 an accomplished botanist. Mr. Francis J. Lewis, of the University 



