350 
Culver—The Erosive Action of Ice. 
Professor Pumpelly 24 in a paper on secular rock, disintegra¬ 
tion remarks that the plane marking the boundary between dis¬ 
integrated rock and still hard rock mast be an exceedingly ir¬ 
regular one. If we could imagine the loose, altered rock re¬ 
moved where this process has been active in depth, the surface- 
exposed would present a remarkable topography, one in which 
the hardness of the material would play no causative part. The 
prominences would consist of those rocks most resistant to car¬ 
bonic acid,e.g., soft clay slates and mica schists, as well as the 
hard quartzites and sandstones. 
The depressions would represent rocks more or less easily acted 
upon by carbonic acid, water and oxygen, and to some extent 
all rocks carrying feldspar in abundance. 
These depressions are largely closed basins . . more or less 
filled with the decayed rock until emptied by ice, wind, or some 
other agency. 
He regards this disintegrated material as the chief source of 
the glacial drift and says he would measure the excavating 
power of the continental glacier, in solid rock by only a small 
portion of the finest part of the glacial debris. 
[It is proper to state here that Professor Chamberlin has al¬ 
ways held that the mechanical origin of the till was clearly in¬ 
dicated by the presence of carbonates in it. Residual clays are 
notably free from carbonate.] 
One of the strongest advocates, of the great erosive power of 
ice was the late Professor Newberry. He believed that ice was 
a much more effective agent than rivers. 
Some of his latest papers were devoted to this subject. It 
was his belief that even the great American lakes had been 
largely formed by ice action. 
Of late years quite a number of geologists have spent consider¬ 
able time in studying the work of living or present glaciers. 
Of these may be mentioned Professor J. W. Spencer, who 
thus writes of his observations on the Norwegian glaciers: 
“Many of the glaciers are advancing and are seen to arcii 
over from rock to rock, leaving sub-glacial caverns into which 
the explorer can go long distances. 
24 Am. Jour. Sci., vol. 17. 1879. 
