55 



which the excentricity of the earth's orbit was unusually great. 

 Of course, the excentricity, and with it the intensity of the 

 recurrent glacial periods, increased and decreased gradually; and 

 the glacial periods which occur while the excentricity is attain- 

 ing, or has passed, its maximum, and which consequently never 

 attain a maximum of glaciation, are what I call rainy periods. 

 In strictness, the title of this paper ought to be " The rainy, or 

 pre- and post-glacial periods"; but I say post- rather than pre- 

 glacial, because pre-glacial deposits, or any evidence of the pre- 

 glacial state of things on the surface of the earth, are likely to 

 have been destroyed by the glaciers of the period of maximum 

 glaciation ; while the surface of the earth has been in many 

 places preserved just as it was when the glaciers of the last 

 glacial period disappeared. 



As a rainy period was only an imperfectly developed glacial 

 period, it follows that, in each hemisphere, rainy periods were 

 alternated with dry or at least not-rainy periods ; and I have 

 now to state what appears to be geological evidence of this. 

 Professor Blytt, of Christiania, appears to have discovered in 

 the great bogs of his own country evidence of alternate wet and 

 dry periods of considerable length, and beginning from the 

 subsidence of the last glacial period. The following is from 

 Nature, of 29th December, 1881 : — 



" In a recent number of Naturen, Prof. Axel Blytt concludes 

 the highly interesting series of papers in which he has at =ome 

 length expounded his theory of the immigrations into Norway, 

 of different floras, during early dry and wet periods. On care- 

 fully examining the oldest Norwegian turf-bogs, he finds, as 

 Prof. Steenstrup has shown in Denmark, that four distinct 

 turf layers may be traced, between which there are frequently 

 two or even three equally distinct deposits, composed of the 

 roots and other remains of trees. The latter are found in situ, 

 and by the undisturbed condition of the turf-beds above and 

 below them, they afford a conclusive proof that such severed 

 trunks cannot have been cut down by human agency. These 

 separate tree-beds the author regards as mementoes of long 

 periods of dryness, which may have endured for thousands of 



