66 J. W. SPENCEB — Illi. II CONTINENTAL ELEVATION. 



period. To such an extent has the ancient valley or canon been filled, first 

 with drift, and this covered with river alluvium, that its original rocky floor 

 is now buried to a depth of 170 feet, even at La Crosse, a thousand miles from 

 the Gulf of Mexico. Farther south the depth of these loose deposits in- 

 creases, until at New ( Orleans a boring of 6.30*)" feet below sea level does not 

 penetrate the southern drift, nor even reach to its lowest members. The 

 lower 500 miles of the ancient Mississippi were excavated out of Eocene or 

 Cretaceous deposits, whilst the valley above the mouth of the Ohio has 

 the form of a canon, excavated out of Paleozoic rocks, varying in width 

 from ten to two <>r three miles, and having a depth (exclusive of the portion 

 now filled) of from 150 to 550 feet, according to the late General G. K. 

 Warren. 



From this inspection of the river, it is easily seen that no natural rainfall 

 could so increase the volume of the discharge as to remove all the deposits 

 which now fill the old valley, much less excavate the original and immense 

 canon. A vastly greater elevation of the continent was necessary. Even 

 were the whole continent uniformly elevated 630 feet, together with the re- 

 mainder of the unknown depth of the ancient Mississippi river, at New 

 Orleans, the canon of the upper part of the river would require a still 

 greater relative elevation of the northern country in order to give sufficient 

 channeling power to the flowing waters; but the slope of the floor of the 

 partially buried valley is much less than that of the modern, as was formerly 

 shown by the author.]; Here, again, is the proof that the country drained by 



the upper waters of the Mississippi once st 1, relatively to that in the region 



of its mouth, much higher than at present. Of the amount, which was at 

 least many hundreds of feet, we have no absolute measurement; nor can we 

 ascertain it by calculation, for there is no register of the excess of the amount 

 of rainfall during the epoch of the greatest sculpturing over that of the 

 presenl day. 



Whilst these records of the Mississippi, which have been only partially 

 deciphered, do not furnish all of the desired information, yet as far as they 



•_'.. they arc invaluable. 



Passing from the buried channel of the Mississippi to its continuation, now 

 submerged beneath the waves of the Gulf of Mexico, we find evidence indi- 

 cating such a stupendous continental elevation as to be almost incredible, 

 were ii doI supported by collateral evidence, upon both the Pacific and 



antic coasts The Boundings off" the coast of the delta of the Mississippi in- 

 dicate the ouier margin of the continental plateau a< submerged to a depth of 

 3,600 feet, indented by an embayment of another hundred fathoms in depth, 

 at the head of which there is a valley a few miles wide, bounded by a plateau 



. ol, [,1883 ; 

 w. B Heard, Km. Jour. Be, 2nd Ber., Vol. XI. \ III, 181 I, p .'.33. 

 I Am. .Nut., Vol. XXI, 1887, pp. 168-7L 



