542 CENOZOIC TIME. 



the cooling effects from it may have been added to, those from the increase of northern 

 lands. 



(3.) An increase of moisture in the air, and therefore of precipitation, this being suf- 

 ficient without a northern elevation, and without an increase of cold beyond the present 

 — a view that is at variance with the fact that the average amount of precipitation 

 over different regions is one of the constants in nature, not alterable except by a change 

 either in the level of the land, or in the courses of the oceanic currents; and that any 

 change in the currents except that from elevating northern lands would tend to diminish 

 rather than increase evaporation. A northern submergence, while it might increase the 

 amount of precipitation, would raise also the mean temperature, by opening the Arctic 

 more broadly than now to the tropical Oceanic currents, and so prevent a southward ex- 

 tension of an ice-mantle ; and just this took place in the Champlain period or era of 

 submergence. 



(6.) Diminished heat in the Sun, on the hypothesis that the Sun has its long-period 

 cycles of maximum and minimum heat. The action of this cause would make cool 

 tropics, along with the cold Arctic regions. 



3. Exterminations and migrations consequent on the approach of 

 the cold period. — The approach of the cold Glacial era probably pro- 

 duced that extermination of species which closed the Tertiary age, be- 

 sides causing the migration to more southern latitudes of species not 

 exterminated. Some facts illustrating the latter point are mentioned 

 on pages 532, 533. The former hardly needs illustration. The cold 

 must have come on with extremely slow progress. The extermina- 

 tion of the terrestial Tertiary mammals, or such as did not find shelter 

 to the South, may have been an early effect of the progressing refrig- 

 eration ; and, long before the cold had covered the continent with its 

 ice-cap, species adapted to a more rigorous climate, that is, those of 

 Quaternary times, may have begun to occupy the country. 



The Glacial period, which is here shown to have been, in all prob- 

 ability, an era of high latitude elevation, was followed by one of un- 

 questioned depression — the Champlain period. 



2. CHAMPLAIN PERIOD. 

 1. American. 



The Champlain period is so named from the occurrence of beds 

 of the period on the borders of Lake Champlain. 



The term Champlain is applied to marine deposits of the period by C. H. Hitchcock, 

 in the Report on the Geology of Vermont. 



I. General Course of Events. 



The earlier part of the Champlain period was the era of the 

 melting of the great glacier, and of most local glaciers ; and therefore 

 the era of immense fiords along the valleys ; of many and great lakes ; 

 and of the deposition of the sand and gravel of the glacier, except 

 the relatively small part which had been earlier dropped. "While the 



