Relation of Secular Decay of Rocks. — Tarr. 37 
Chamberlain and 8alisl)uiT, speaking of the loess of Wisconsin* 
describe it as neither a sand nor a clay, but intermediate between 
the two, being generally coarser than residuar}' earths. The par- 
ticles are, however, angular and irregular as in the case of resid- 
uary earths. Its depth in that region is from ten to fifteen feet 
over a wide area. 
These considerations are in the main theoretical, l)ut the theory 
has much to commend it, since it accounts, apparently in a satis- 
factory manner, for a most puzzling series of deposits. 
{d) By Ice. 
The movement of a sheet of ice over a region which had been 
subjected to secular disintegration would result in the removal of 
the loose, decayed rock, which would furnish to the ice great 
quantities of sediment. During the melting of the ice, and at all 
times at its front the streams would be furnished with great bur- 
dens of sediment, which, either deposited in the valleys or car- 
ried to the sea, would aid greatly in rock formation. The final 
disappearance of the ice would leave the surface of the country 
littered with unstratified drift. 
Something of the same condition would result from the passage 
of an ice sheet over an undecayed rock, but the materials supplied 
would be less in amount. Ice cannot cut without tools. Leaving 
out of consideration valley glaciers, t there are but two waj's with 
which ice can be supplied with cutting tools, one by the loose ma- 
terial which it can pick up, the other l)y actually rending rocks 
asunder. The former supply ceases when the zone of disintegra- 
tion is passed and the latter soon follows. The erosive action of 
ice is to round, smooth and polish the surface over which it passes 
and thus to lessen the possibility of obtaining a supply of cutting 
tools. As the period of ice occupancy of a land continues, its 
power of erosion must diminish and finally become almost noth- 
ing. It will slide over rounded surfaces practicalh' without any 
destructive effect and the streams will issue from its front with a 
very slight load of sediment. 
The great mass of detritus left by the ice of the glacial period 
*Driftless area of "Wisconsin. Sixth Annual Rept. U. 8. Geol. Survey, 
278-307. 
tin valley glaciers and in continental glaciers where mountain peaks 
project above the ice, detritus is furnished as a talus from the cliffs. 
