430 GEOLOGY. 



probable sources. To allow time for the residue of winter snow above 

 summer melting to build itself up to a height capable of giving effective 

 motion, and then to allow time to carry drift this great distance at 

 any probable rate of motion, taxes the hypothesis very severely to 

 say the least, for a high rate of motion probably cannot be assigned 

 safely. 



There is a widespread misapprehension as to the average rate of 

 movement of the ice-fields of Greenland, which are almost our only 

 available field of observation on the motion of continental glaciers. 

 In certain fiords that lead out from great basins into which broad 

 fields discharge their ice and their surface waters, and thus furnish the 

 conditions for an extraordinary rate of movement, the rate of motion, 

 at least during summer, is unusually high, and these exceptional cases 

 have been taken as representative of the movement of the border 

 of the inland ice. This is very far from being true. The average 

 movement for the whole border of the ice field is quite certainly less 

 than one foot per day, and is more likely less than one foot per week. 

 The melting and evaporation at the edge of the ice fields of Greenland 

 cut it back only a few feet per year, because of the shortness of the 

 season and the covering of annual snow. Probably the wastage does 

 not reach ten feet per annum. It is certainly much less than 10 feet 

 in northern Greenland. If 12 feet be allowed for this, there should 

 be an average advance of the edge of the ice of 40 feet, on the basis 

 of one foot per week onward movement. This amount of advance for 

 the 1400 to 1600 miles of ice border tributary to Baffin's Bay, would 

 require the discharge of more than 1000 icebergs annually, averaging 

 100 feet in length and 300 feet in breadth, to remove the excess of ice 

 and keep the margin of the ice-fields stationary, and this number of 

 icebergs of these average dimensions exceeds the estimates of Rink 

 and others. If the estimate were raised to one foot per day, the num- 

 ber of discharging icebergs would obviously greatly exceed the observed 

 number. If the rate of advance be approached from the point of 

 view of precipitation, computations show that either an enormous 

 snowfall over vast regions or an almost total absence of melting and 

 evaporation must be postulated to account for the building up of the 

 great Pleistocene ice-sheets, and for developing their observed radial 

 movements within such limited periods of time as the Crollian hypoth- 

 esis requires. 



