SURFACE GEOLOGY. 213 
lakes are but portions of great excavated channels locally 
filled with drift material, the fiords of the northern Atlantic 
and Pacific coast present remarkable parallels to them ; and 
I would suggest Puget's Sound, Hood's Canal, and other 
portions ‘of that wonderful system of navigable channels 
about Vancouver’s Island, as affording interesting and in- 
structive subjects for comparison. Like our lakes their 
channels are for the most part excavated from sedimentary 
strata which form a low and comparatively level margin to 
the bases of mountain chains and peaks. They too have 
their depths and shallows, their basins and bars, and probably 
all who have seen them will assent to Professor Dana’s view, 
that they are the "result of subaérial excavation," in which 
glaciers performed an important part. 
The " Loess” of the Mississippi Valley. The “Bluff form- 
ation" of the West, sometimes called "Loess," from its re- 
semblance to the Loess of the Rhine, I have on a preceding 
page designated as a lacustrine non-glacial Drift. deposit. It 
seems to be the sediment precipitated from the waters of our 
great inland sea.in its shallow and more quiet portions, to 
which icebergs, with their gravel and boulders, had no ac- 
cess, and where the glacial mud was represented only by an 
impalpable powder, which mingled with the wash of the 
adjacent land, land shells, etc. 
It is evidently one of the most recent of the deposits 
which come into the series of Drift phenomena, and was ap- 
parently thrown down while the broad water surface which 
once stretched over the region where it is found was narrow- 
ing by drainage and evaporation, till, by its total disappear- 
ance, this sheet of calcareous mud was left. 
It underlies much of the prairie region, and once filled, 
often to the brim, the troughs of the Mississippi and Mis- 
souri, so deeply excavated during the glacial epoch. When 
the system of drainage was re-established the new rivers be- 
gan the excavation of their ancient valleys in the Loess. 
When they had cut into or through this stratum, so that it 
