242 The American Geologist. October, 1902. 
Greenland g^laciers is estimated at 9230 tons daily * and still 
another recorded example is that of Justedal glacier of Norwa)'- 
which discharges according to A. Helland, such an amount 
that its 830 square miles of ice field area loses approximately 
69000 cubic yards of solid rock every year. The amount of 
such work done by glaciers where based upon the data given 
by sediment discharged may, however, in some cases be er- 
roneous since it cannot always be shown just how much of the 
sedimenc does not come from material washed in by ice 
streams, due to springs and melting snows. 
Professor Penck has calculated with r'^ference , to the 
ancient glacier of the Isar, that the amount of its erosion has 
been such that over forty-two feet of rock have been eroded 
from a surface of 1,080 square miles in extent. Furthermore 
his estimate of the erratic material removed from the Alps and 
scattered over the "Vorland" between the IlVr and the Inn 
is equal to about 880 cubic miles, and weighs approximate- 
ly 1,000,000,000,000 tons. The same author referring to ob- 
servations on the Unter Aar, says that the mud discharged 
per year as a result of glacier action would lower its bed by 
three feet in 1,666 years, to do which by rapidly flowing water 
would require 4,125 years. This indicates that the glacier 
erodes more than twice as rapidly as running water. This 
comparative value of the erosive effect of ice and v/ater is con- 
servative, because in the case of some of the larger glaciers 
the difference is estimated to be much greater in favor of the 
ice. 
The result of glacial erosion in the economy of nature must 
be regarded as ver>' important ; it is evidenced in the modifi- 
cation of topography over extended areas, and furnishes also 
a reflection of their history. 
"The basin at the bottom of a cirque," says James Geikie, 
"is the work neither of running water nor of frost alone, but 
has been ground out by glacier ice." * Glacial lakes also 
frequently mark the track of old glaciers, the basins having 
been produced by glacial erosion. In Finland, a land of lakes^ 
evidence is in abundance that the basins of these lakes were 
the work of the inland ice of north Europe. Likewise in 
North America a vast number of the smaller lakes are directly 
*Meddelser om Gronland, Vol. 2 (Geikie.) 
tEarlli sculpture, p. 289. 
