Glacier JJ^ork. — Scott. 249 
the probable glacial epochs, and the extension of glacier work 
during Pleistocene and Carboniferous times. 
The efficiency of the agencies whereby carbon dioxide is 
withdrawn from and subsequently restored to the atmosphere, 
is shown to be correlated with geographic conditions, includ- 
ing the distribution of land and sea, and continental elevations. 
Since the geography of the earth in Paleozoic time is so im- 
perfectly known, the hypothesis does not presume to so com- 
pletely explain glaciation of that era, though the author presents 
and discusses such facts as are evident. To account for the 
depletion, of the atmospheric carbon dioxide during this period, 
the locking up of the carbon as coal is considered most impor- 
tant, and it is shown that glaciation would result after a longer 
continued depletion had taken place, than was true of Pleisto- 
cene time. * 
Work of the Ice Sheet. 
The work of the great ice sheet as a powerful agent in 
shaping topography over portions of the globe appears to admit 
of division into two main parts : Erosion and Transportation, 
The two parts are obviously more or less interlocked with 
each other, for erosion means transportation of material for at 
least some finite distance, and transportation is materially nec- 
essary to produce erosion. The division is here made, however, 
as covering the great results which are in evidence, and tlie two 
parts will be considered as such only in reference thereto. 
In considering the subject of erosion by the ice sheet, it is 
well to consider first the work of the pure ice itself. Evidence 
is conclusive that it possesses little abrasive power, and prac- 
tically no striating ability. It neither grooves nor scratches 
the bed rock, and can at most only very slightly wear it down. 
It is obvious that whether widespread erosive results be con- 
sidered to be due, as thought by some, to icebergs floating in 
a sea covering the areas in question, or due to the oscillations of 
a great ice mass over the surface, or both, the existence of a 
massive field of ice at some time is unquestionable. It is also 
probable that an irregularity of topographic conditions pre- 
ceded the Glacial period, for there is no obvious reason whv 
disintegration and decomposition by mechanical and chemical 
*NOTE. — For an excellent brief review of Prof, diamberlln's Paper, 
see "Climate iind Carbonic Acid" by Bailey Willis, U. S. Geol. Surv., 
Pop. Sci. Mo. July, 1901. 
