THE REIGN OF ICE. 271 



had scored a ravine which split the land. The streams were 

 inaccessible and dwarfed, and their availability for human 

 uses seriously impaired. There must be a general repair of 

 the surface before it would meet the demands of a being of/ 

 such enterprise and resources as man was destined to be. 



The end so necessary was accomplished without departure 

 from the fundamental method of all the previous history of 

 the continents. Uplift and subsidence accomplished the gla- j 

 cial renovation which now approached. We have already 

 studied many indications of glacier action. We have con- 

 cluded (Talk III, which should now be reviewed) from the 

 inductive evidence, that a continental glacier has some time, 

 brooded over the land, and we have made some observa- 

 tions' on actual glaciers (Talk IV). We will now attempt to 

 sketch the glaciation of a continent, from a historical point 

 of view. 



The mild climate of the middle and later Tertiary time \ 

 which had prevailed as far north as Disco, on the coast of 

 Greenland, and Melville and Bennett Islands in the Arctic 

 Ocean, had already been succeeded by a colder one. The cause j 

 of the change remains an unsolved problem. The later invasion 

 of severe cold, throughout the northern temperate zone, is 

 generally ascribed to northern elevation ; but there is much 

 reason to suppose it the result of certain astronomical changes, 

 and to hold, also, that this was but one of a succession of glacial 

 visitations. Whatever the cause, the reality of the glacier period 

 can not be questioned. The area of perpetual snow had ex- 

 tended its limits from the arctic zone into northern America. 

 In the middle latitudes, an unwonted chill was already ex- 

 perienced in the atmosphere. Successive winters grew more j 

 and more severe, and the snow lingered always later in the 

 spring. There were deep ravines where it survived the sum- 

 mer. With continued depression of mean temperature, the 

 winter snows still further delayed their departure. The forest 

 was changed. One by one, the species suited to a milder cli- 

 mate perished ; and frost began to brown foliage in a zone 

 which had witnessed a state of perpetual verdure. Year by 



