202 SURFACE GEOLOGY. 
by Humphreys and Abbot in their splendid study of this 
river being but repetitions of the phenomena exhibited at 
the falls of the Ohio—the river running over one side of its 
ancient bed. 
The trough of the Mississippi is not due to synclinal struc- 
ture in the underlying rocks, but is a valley of erosion sim- 
ply. Ever since the elevation of the Alleghanies—7.e. the 
close of the Carboniferous period — it has been traversed by 
a river which drained the area from which flow the upper 
Mississippi, the Ohio, the Tennessee, etc. Since the Mio- 
cene period, the Missouri, Arkansas, and Red rivers have 
made their contributions to the flood that flowed through it. 
The depth to which this channel is cut in the rock proves 
that at times the viver must have flowed at a lower level and 
with a more rapid current than now ; while the Tertiary beds 
formed as high as Iowa and Indiana in this trough, and the 
more modern Drift clays and boulders which partially fill the 
old rock cuttings, show that the mouth and delta of the river 
have, in the alternations of continental elevation, travelled 
up and down the trough at least a thousand miles; and that 
not only is it true, as asserted by Ellet, that every mile be- 
tween Cairo and New Orleans once held the river's mouth, 
but that in the several advances and recessions of the waters 
of the Gulf the mouth has been more than twice at each 
point. The change of place of the delta has been caused, 
however, for the most part, by oscillations of the sea level, 
and not, as Ellet supposed, by the filling of the channel by 
the materials transported by the river itself. 
Dnrrr Deposits. The Drift deposits which cover the gla- 
cial surface, consisting of fine clays below, sands and gravel 
above, large transported boulders on the surface, and the 
series of lake ridges (beaches) over all, form a sequence of 
phenomena of which the history is easily read. 
Erie Clays. The lower series of blue or red clays—the 
"Erie clays” of Sir William Logan— over a very large area, 
rest directly on the plain and polished rock surfaces. These 
