344 W. UPHAM — GLACIAL ACCUMULATION AND INVASION. 



tide highlands north of the Saint I^awrence river, over the hasin of James 

 and Hudson bays, on the country extending westward to the Athabasca, 

 Reindeer and Winnipeg lakes, and west of the Rocky mountains on the 

 northern half of British Columbia, becoming upon these tracts so thick 

 that its borders advanced outward, invading the northern United States, 

 and reaching the extreme limits of their farthest incursion along the 

 southern boundary of the drift. According to this view, an ice invasion 

 or two or more invasions at successive times extended the thick land ice 

 of Canada southward hundreds of miles to the Missouri and Ohio rivers, 

 the advance of its border being attended with all the exceptional condi- 

 tions of glacial erosion and deposition which prevailed upon a belt sev- 

 eral or many miles wide next to the ice margin and along that terminal 

 line. 



Instead of this view, it is the purpose of the present paper to call atten- 

 tion to evidences that the ice-sheets were principally accumulated by 

 snowfall on all their area, coming into existence as the snow of a great 

 winter storm spreads its white mantle simultaneously over the northern 

 United States and Canada. Such snowfall, not wholly melted away in 

 the summers and preserved with increasing depth during centuries and 

 thousands of years, is here regarded as the origin and cause of growth 

 of the ice-sheets throughout all their extent. To this source of ice forma- 

 tion and increasing supply the outer portions of the ice-sheets were prob-' 

 ably due in only slightly less measure than their inner and central por- 

 tions. During the time of accumulation of both the North American 

 and European ice-sheets the precipitation of snow by storms sweeping 

 over them was doubtless greatest on their windward borders to a distance 

 of 100 or 200 miles inward from the margin of the snow-covered area — 

 that is, upon the belt which the storms passed over during their first 

 three to five or ten hours after leaving the sea or land and coming on the 

 snow and ice. Enough precipitation, however, took place farther on to 

 build up the central areas of the ice-sheets higher than their peripheral 

 tracts, and so to produce an outward flow of the ice on all sides toward 

 its margin, with erosion and transportation of drift from even the central 

 districts to the boundaries. As the rate of ice accumulation was greatest 

 not far back from the edge, the rate of glacial outflow and the slopes of 

 the ice surface there were likewise greater than upon its central tracts. 



THIN MARGIN AND LOW GRADIENTS OF THE ICE-SHEETS. 



That the outer parts of the ice-sheets were not brought by an incursion 

 but were amassed by snowfall is indicated by the evidences of their 

 gradual attenuation. The extreme limits of the North American and 

 European glacial drift are generally marked by no conspicuous morainic 



