Mat 1902.] BOTANICAL SOCIETY OF EDIXBUKGH 245 



the native flora, each section in its own place ; then came the 

 Boreal and Arctic floras, a large part of which must have 

 presented a facies such as would be met with in the 

 Siberian Tundras ; next, but ouly on the south-east margin 

 of the chief area occupied by ice and snow, followed the 

 Steppe flora. It would not be very difficult to mark these 

 belts on a map where these zones of vegetation occurred at 

 the culmination of the Age of Snow — perhaps it has 

 already been done ? 



The enormous amount of glacial erosion which was 

 accomplished while the North Sea was occupied by land ice^ 

 proves conclusively, to my mind at least, that the Age 

 of Snow was one of immense length. It may well have 

 taken up the larger part of the time between the Forest 

 Bed period and now. Hence, as the time occupied in these 

 changes of station was so long, and the process of both 

 expatriation and repatriation was so gradual, one need 

 hardly wonder at the small amount of change that has 

 ensued. 



To understand what followed after the climax of the 

 Age of Snow was reached, we shall need to adopt some 

 working hypothesis regarding the cause of this remarkable 

 episode : — It is by no means necessary to suppose that the 

 Age of Snow was characterised by a low temperature. 

 All that is needed to account for the facts is to postulate 

 such geographical conditions as should lead to a little — 

 perhaps only a very little — more snow falling on the low- 

 grounds each year than the heat of summer (which may 

 have been considerable, even in Scotland) snfiiced to melt. 

 To make the snow, in the first instance, there must have 

 been — (1) copious evaporation going on over the Atlantic; 



(2) there must have been aerial currents to transport the 

 resulting aqueous vapour in the direction required ; and 



(3) refrigerators, in the shape of snow-covered areas, 

 sufficiently powerful to congeal the aqueous vapour 



^ I have endeavoured in several papers to explain how the Norwegian 

 ice crossed the North Sea. Gravitation was but a siibordinate factor. 

 The chief paper is published in the " Royal Physical Society's Proceed- 

 ings," vol. XIV. p. 137, under the title of "Solar Energy in Relation 

 to Ice." 



