SURFACE GEOLOGY. 17 
has never been reached. The trough of the Hudson is deeply silted up, 
as we know by the explorations made at Jersey City and in the Hast 
river. The channel of this stream, as has been shown by Dana, can be 
traced on the sea bottom eighty miles south and east of New York, where 
it once discharged itself at the true margin of the continent, 600 feet 
below the present level of its mouth. The peculiar character exhibited 
by the present outlets of the Delaware, the Potomac, and James rivers 
indicates that they also, like the Hudson, once entered the Atlantic much 
farther east than now, and that their old mouths are completely buried 
and obliterated. 
The lower Mississippi bears unmistakable evidence of being—if one 
may be permitted the paradox—a half-drowned river; that is, its old 
channel is deeply submerged and silted up, so that the “father of 
waters,” lifted above the walls that formerly restrained him, now wan- 
ders lawless and ungovernable whither he will in the broad valley. 
The thickness of the delta deposits at New Orleans is variously re- 
ported from 1,500 feet upwards, the discrepancies being due to the diffi- 
culty of distinguishing the alluvial clays from those of the underlying 
cretaceous and tertiary formations. It is certain, however, that the bot- 
tom of the ancient channel of the Mississippi has never been reached 
between New Orleans and Cairo; the instances cited 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 structure in the 
underlying rocks, but is a valley of erosion simply. Ever since the ele- 
vation of the Alleghanies—~z. 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 Miocene 
period, the Missouri, Arkansas, and Red rivers have made their contri- 
butions to the flood that poured through it. The depth to which this 
channel is cut in the rock proves that at times the river must 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 bowlders which partially fill the 
old rock cuttings, show that the mouth and delta of the river have, in 
the alternations of continental elevation, traveled up and down the trough 
at least a thousand miles; and that not only is it true, as asserted by 
Hllet, that every mile between 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. 
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