730 H. M. BANNISTER 
diameter and one in which no reasonably supposable elevation 
could give a uniform slope varying appreciably from the hori- 
zontal. An elevation at the center of radiation of ten thousand 
feet (which is much beyond the most favorable interpretation 
which any known data will bear) with an ice-cap of as much more, 
would only make a slope of under half a degree in eight hun- 
dred miles, and of considerably less in some directions to the 
outer limits of the ice. Some glacialists, however, are liberal in 
their allowances of earth movement to account for the flow of 
the glacier. Mr. Upham," for example, thinks that the strong 
current needed to transport bowlders from the southeast shore 
of Hudson Bay one thousand miles southwestward to southern 
Minnesota, would require a slope of at least fifty feet or more 
per mile, apparently unmindful of the fact that such a slope for 
the given distance would require an elevation of the ice-cap to 
the height of 50,000 feet, where precipitation would, if it 
occurred at all, probably be so slight as to seriously embarrass 
the formation of any considerable ice-cap whatever. It is not 
probable, however, that there was any uniform slope over the 
glacial field, and whatever effect was produced by gravitation 
could not be such as would cause a rapid motion of the ice, 
“faster than the Swiss glaciers.”* The other theories that have 
been invoked for the glacier motion, the effects of thawing and 
freezing, expansion under varying temperatures, etc., are none 
of them, we think, counted as sufficient to cause rapid movement 
in so large a mass, as a whole, even by their upholders, and such 
estimates as two to five feet per day are hardly based upon a 
due consideration of the probable or possible physical condi- 
tions. Dana’s? estimate that ‘the rate of motion could hardly 
have exceeded a foot a day, and may have been in most parts 
no more than a foot a week”’ is much more likely to be near the 
truth. Ice, except under special conditions of pressure or vis a 
tergo, barely moves ona slope of one degree, and an average slope 
"Greenland Ice Fields and Life in the North Atlantic, p. 304. 
°? WARREN UPHAM. Bull. Geol. Soc. Am., III, 401, 1892. 
3 Geology, 3d ed, p. 539. 
