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 East 

 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 a.nd 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 — i. 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 

 Ellet, 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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