AND THE SALT DEPOSIT ON PETITE ANSE ISLAM). 31 



Louis to within a few miles above Cairo, is manifestly a modern erosion ; the hu^c 

 masses of (siliceous) carboniferous pebbles bordering the Tennessee on the west 

 and reaching far down into Alabama, speak of great degradation of beds foriniu"- 

 the southern edge of the carboniferous basin; and the divergence of the great Drift 

 delta from the head of the ancient Mississippi embayment, is itself a fact of no 

 small cogency. I trust that a thorough geological and topograpliienl examination 

 of this interesting region will soon enable us to come to more definite conclusions 

 regarding its history during the Drift epoch. 



THE "TERRACE" EPOCH. 



The precise extent of the maximum depression of the Gulf States at the end of 

 the Champlain period of depression, must be determined by more accurate 

 measurements than we now possess. Yet we may approximate to a minimura. 



Commencing at a level now at least four hundred and fifty feet below the present 

 tide-water (as shown in the Calcasieu bores), the depression, slow at ilrst and more 

 rapid afterwards, continued until the summits of the highest hills bearing aqueous 

 deposits formed in quiet water (/. e. not by rapid streams) were, not merely at, bat 

 some depth beloiv, the Gulf level; it being remembered that the pr^'sent elevation 

 of these hills, degraded as they have been for ages by atmospheric agencies, do not, 

 themselves, represent their original maximum height, perhaps by many feet. 



The highest elevation now existing on the banks of the Mississippi Ei^-er, viz. at 

 Fort Adams, near the line between Mississippi and Louisiana, h about three 

 liundred and fifty feet above tide-water. But summits obviously higher are visible 

 from this point, southeastward ; and there is a decided ascent as we proceed inland 

 from the river, so as to render it probable that the present maximum elevation of 

 the Loess deposits proper, above tide-water, is not less than four hundred and fifty 

 feet in the latitude of Fort Adams. 



But the Loess is overlaid (wherever denudation has not removed it) by the 

 yellow Loam stratum, which exists equally in the interior of the Gulf State?, on 

 all but the very highest ridges. It is Avidely developed in Tsorih Mississippi and 

 West Tennessee, and according to the railroad surveys, much of this region is 

 elevated between five hundred and fifty and seven hundred feet above the Galf 

 In view of these facts, I think 07ie thousand feet the minimum at which v/e can 

 estimate the entire amount of depression during the Champlain epoch, from data 

 existing south of Cairo. It would not require much more to transform the prairies 

 of Illinois into the fresh-water glades to which Mr. Lesquereux attributes their 

 origin; and Prof E. B. Andrews' observations in southern Ohio seem to justify 

 the assumption of a depression greater by several hundred feet than the minimum 

 called for by mine. 



The reverse motion during the Terrace epoch has, of course, compensated the 

 greater portion of the depression; at least, all that part remaining as a difference 

 after deducting, substantially, the maximum depth at which drift materials 



