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



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 

 observed upper limits of glaciation on high mountains, Prof. 

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

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

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

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

 Laurentide highlands, James Bay, Lake Winnipeg, Reindeer and 

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

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

 probably buried beneath the ice expanse. In British Columbia, 

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

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

 depth. 



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

 nental glacier flowed outward, carrying its drift from Hudson 

 Strait, Labrador, and Newfoundland easterly beyond the present 

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

 easterly across New England, and southerly and southwesterly 

 across the basins of the Laurentian lakes ; from Manitoba and 

 the Saskatchewan region southerly into Minnesota, Iowa, the 

 Dakotas, and Montana; from British Columbia into Idaho and 

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

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

 the great northern Barren Grounds northerly down the Macken- 

 zie and across the islands of the Arctic Sea. 



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

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

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

 as that of North America ; and its maximum depth above Sweden 

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

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

 neled Scandinavian plateau even now has numerous local ice 

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

 doubtless remnants of a continuous glaciation through all the 

 centuries 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- 

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

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

 arrangement. 



