198 SURFACE GEOLOGY. 
and Tennessee was arrested, their channels filled, terraces 
formed, etc. If the Upper Tennessee has, as appears, a 
channel lower than the Mussel Shoals, it must be somewhere 
connected with the deep channel of the lower river. 
It should be said, however, that it by no means follows 
that where an old earth-filled channel passes around the 
rocky barrier by which the navigation of our rivers is im- 
peded, it will be most convenient and economical to follow 
it in making a canal to pass the obstacle, as the course of 
the old channel may be so long and cireuitous that a short 
rock cutting is cheaper and better. The question is, how- 
ever, of sufficient importance to deserve investigation, before 
millions of dollars are expended in rock excavation. 
If it is true that our great lakes can be connected with each 
other and with the ocean, both by the Hudson and Mississippi, 
by ship canals, —in making which no elevated summits nor 
rock barriers need be cut through, — the future commerce cre- 
ated by the great population and immense resources of the 
basin of the great lakes may require their construction. 
3d. Upon the glacial surface we find a series of unconsoli- 
dated materials generally stratified, called the "Drift de- 
posits." 
Of these the first and lowest are blue and red clays (the 
Erie clays of Sir William Logan), generally regularly strati- 
fied in thin layers, and containing no fossils, but drifted 
coniferous wood and leaves. Over the southern and eastern 
part of the lake basin, these clays contain no boulders, but 
towards the North and West they include scattered stones, 
often of a large size; while in places beds of boulders and 
gravel are found resting directly on the glacial surface. 
In Ohio the Erie clays are blue, nearly two hundred feet 
in thickness, and reach up the hill-sides more than two hun- 
dred feet above the present surface of Lake Erie. On the 
shores of Lake Michigan these clays are in part of a red 
color, showing that they have been derived from different 
rocks, and they there include great numbers of stones. 
