W. Hilgard—Geology of the Delta, 



with in 



It is, doubtless, owing to the formation of these swamp depos- 

 its, and their subsequent resistance to denudation during and 

 since the Terrace epoch of elevation, that the main body of the 

 truly alluvial delta is thrown so far beyond the general coast 

 line,t out into the Gulf. It is the shallow ' ' blue clay bottom;' 

 so well known to navigators on the Grulf coast, which forces the 

 great river to advance its mouths so rapidly toward deep watrr. 

 by the accumulation of its own deposits ; and the borings m-A^ 

 at New Orleans have shown how slight is the thickness, even at 

 such an advanced point, of the river deposits proper, overlying 

 the older formation. 



I owe to the active interest taken in this subject by Gen. A. 

 A. Humphreys, XJ. S. A., an opportunity of examining, not 

 only the specimens collected during the boring of the artesian 

 well at New Orieans by a committee of the New Orleans Acad- 

 emy of Sciences (so far as they were preserved from destruc- 

 tion during the war) ; but also those obtained in the soundings 

 made by the delta survey under his charge, upon which a verv 

 able and minute report of a microscopic examination had previ- 

 ously been made by Mr. L. F. Pourtales. Upon the strength 

 of the data furnished by the latter, as well as by the profile con- 

 structed by the committee of the Academy (reproduced m the 

 "Report on the Physics and Hydraulics of the Mississippi 

 river"). Gen. Humphreys concluded that at Bonnet Carre and 

 New Orleans, the river "flows on an ancient sea bottom, whicU 

 he conjectured to be of Tertiaiy age. Sir Charies Lyell hav- 

 ing questioned the correctness of this view. Gen. Humphreys 

 obtained from the N. 0. Academy as complete a suite of speci- 

 mens of the borings as could be collected, and referred them to 

 me for examination. The first results of this investigation are 

 given, in substance, in the first volume of Lyells Princip es oi 

 Geology, 10th edition, p. 459 ; they were based substantia i} 

 upon the determination of the visible shells (mollusks) co^^ 

 tained in several of the specimens, embracing, fortunately, mos 

 of the important horizons mentioned in the profile. 1 Ji^J 

 since gone over the whole ground, in the microscopic examin 

 tion of all the available specimens, with a view to determining 

 their (marine or fresh water) character, and the admissibilit^^"^ 

 the supposition that they might belong to the delta iorma 



My detailed report of this examination will, I P^^^f' !! 

 published before long.:}: Unfortunately, most of the specim*^ 



* Miss. Rep., 1860, p. 156. . .a^<,7.csA 



t Drawn, say from the mouth of Pearl river to Belle 

 outpost of the Port Hudson deposits on the Louisiana cc 

 near the city of New Orleans. 



X In Rep. of the U. S. Engineer Dept., for 1870. 



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