CAUSES, STAGES, AND TIME OF THE ICE AGE. 355 



I five hundred and seventy-five thousand square miles,* and rises 

 with average slopes of one hundred feet or more per mile to a 

 central height, along its axial portion, of eight to ten thousand 

 feet, or almost two miles measured vertically, above the sea level. 

 The ancient ice sheets had a similar altitude and thickness. 

 From the directions of outflow of the North American ice fields 

 as shown by the transportation of the glacial drift, and from the 

 bserved upper limits of glaciation on high mountains, Prof, 

 ames D. Dana estimated the thickness of the ice formerly accu- 

 ulated above the Laurentide highlands, between the St. Law- 

 ence River and Hudson Bay, to be fully two miles. It probably 

 aried in thickness from one to two miles across Labrador, the 

 aurentide highlands, James Bay, Lake Winnipeg, Reindeer and 

 thabasca lakes, to the Rocky Mountains, in the region of the 

 eace River, where their summits, lower than southward, were 

 robably buried beneath the ice expanse. In British Columbia, 

 cording to Dr. George M. Dawson's observations of glacial 

 rise and drift on mountains, the ice sheet exceeded a mile in 

 epth. 



In all directions from its thick central areas the vast conti- 



ental glacier flowed outward, carrying its drift from Hudson 



trait, Labrador, and Newfoundland easterly beyond the present 



oast line ; from the provinces of Quebec and Ontario south- 



sterly across New England, and southerly and southwesterly 



ross the basins of the Laurentian lakes; from Manitoba and 



e Saskatchewan region southerly into Minnesota, Iowa, the 



akotas, and Montana; from British Columbia into Idaho and 



ashington on the south, into the edge of the Pacific Ocean on 



e west, and down the Yukon Valley on the north ; and from 



e great northern Barren Grounds northerly down the Macken- 



e and across the islands of the Arctic Sea. 



Northern Europe and the present basins of the Irish, North, 



altic, and White Seas were covered by an ice sheet which at- 



ined an extent of two million square miles, being half as large 



that of North America ; and its maximum depth above Sweden 



d the beds of the Baltic Sea and Gulf of Bothnia was one mile, 



r more probably two miles. The high, much eroded, and chan- 



eled Scandinavian plateau even now has numerous local ice 



elds, varying in size up to five hundred square miles, which are 



oubtless remnants of a continuous glaciation through all the 



nturies since the vast European ice field of the Glacial period 



* Measured on a map drafted by the author for Greenland Icefields, by Prof. G. Fred- 

 rick Wright and Warren Upham (D. Appleton & Co., 1896). From my chapters in this 

 )k some later paragraphs of the present paper are derived, with condensation and re- 

 ;ement. 



