222 
The American Geologist. October. 1902. 
having a breadth of nearly three miles was rated at fifty feet 
per day, and that of Karajak, four miles broad, at twenty- 
two to thirty-eight feet in twenty-four hours.- * These 
figures show the average rate of motion to be at least ten 
times that of glaciers of the Alpine type. 
"It has been stated as an empirical rule," says Prof. 
Harrv fielding Reid. t "that the velocity of glaciers in 
beds of uniform slope is greatest in the neighborhood of the 
neve-line, and diminishes as we leave it going up or down 
the glaciers." He proceeds immediately to show, however, 
that this rule has very important exceptions. Prof. Heim 
:*: believes that the increased velocity of some Greenland 
glaciers near their ends, accords with the rule by consider- 
ing the ends as the upper part of the glacier. Prof. Reid 
thinks this a mistake as the ends are some distance below 
the neve-line, but that the real cause is due to a lack of sup- 
port in front, which Prof. Heim also mentions. It is consid- 
ered probable that the under part of these tidewater glaciers 
does not move as fast as the upper, otherwise the bergs 
Avould be of comparatively thin sheets instead of immense 
irregular masses of ice. 
There is a general law, however, that the flow is less be- 
low, than in the neve-line: this flow being equal it is stated, 
to the product of the average velocity, by the sectional area, 
by the effective density. The more rapid surface velocity 
often found, being thought by some authors to be due to 
the formation of immense crevasses which reduce the ef- 
fective density. \Miere the glacier bed is of uniform slope 
the velocity and flow are said to increase together, though- 
not necessarily in proportion. 
Glacier Motion. — General Theories. 
Glacier motion, ever since work on glaciers began, has 
been a subject of much interest, of careful investigation, and 
of keen controversy. It has engaged the attention alike of 
both geologists and physicists. De Saussure, as one of the first 
to consider the question, presumed that the weight of the ice 
*Prestwich, Geologj-, Vol 2, Chaptei- 33. 
t Journal of Geology, Vol. 4. ISflii. p. 913. 
JHandbuch der Gletscherkunde, 1885, p. 1(30. 
