The Scottish Naturalist. 207 



trees. The bogs began in many places to dry up, and were 

 gradually overspread by great forests. But this second genial 

 post-glacial epoch, if we may rely upon the evidence of the upper 

 buried forests of our peat-mosses and marine alluvia, was hardly 

 so genial as that of the earlier forest-epoch. In the Norwegian 

 bogs, for example, the lower buried forests consist principally of 

 leafy trees, while those in the upper beds are almost wholly 

 conifers. But that the climate was more favourable to forest- 

 growth than at present, is shown by the vertical and horizontal 

 range attained by the trees of the second forest-epoch. What is 

 true of Norway is equally true of England and Ireland, and, so 

 far as the evidence goes, of Scotland also. It is further important 

 to note that this recurrence of drier and more genial conditions 

 was accompanied by an extension of land-surface. The sea re- 

 treated to a greater distance than at present, the forests of the 

 second epoch growing over wide tracts which are now submerged. 

 But to what extent that elevation of the land, or retreat of the sea, 

 was carried we cannot tell. There is no evidence, however, to 

 show that Britain again became continental. 



To this second forest-epoch succeeded, as before, colder and 

 more humid conditions. Once more the trees became restricted 

 in their vertical and horizontal range ; while, at the same time, 

 peat-mosses increased and overspread wide regions, which were 

 formerly covered by forests. Again, as before, the sea advanced 

 upon the land. It is to this later period that the formation of our 

 lower-level Carse-clays and the 25-30 ft. beach belongs. When 

 these lower-level Carse-deposits are followed up the valleys, they 

 merge into tumultuous river-gravels, which have much the same 

 appearance as those which are contemporaneous with the Carse- 

 beds of the 45 to 50 ft. level. Thus, in the Tay-valley, ancient 

 river-gravels occur at three distinct levels. Those at the highest 

 level belong to late glacial times ; and were laid down during the 

 final retreat of the great glaciers of the Ice-age proper. Then 

 followed a prolonged interval of denudation, corresponding to the 

 genial continental epoch, when the late glacial gravel-terraces 

 were eroded, and often entirely demolished. To this epoch suc- 

 ceeded one of colder and more humid conditions, when the sea 

 advanced to the 45-50 ft. level, and the tumultuous river-gravels 

 of the second terrace were formed. Again the sea retreated, 

 milder conditions ensued ; and the second river-terrace suffered 



