AND THE SALT DEPOSIT OX PETITE A :< S E ISLAND. 27 



buried cypress swamps so common on the Mississippi, llwjrc. stumps, five to ei"-lit 

 feet high, have their roots buried in a stratum of brown clay continued below water 

 level, twenty-four feet below the top of the bank. Their tops are surrounded by a 

 simdar clay stratum, while a yellow silt envelops the middle portion, and a reddish 

 loam, possibly modem deposits, forms the superincumbent bed, to the surfae(>. 



Similar profiles are found in the lake region lying chiefly north of the main river, 

 and not now forming a portion of its immediate valley; blue clays, howe\er, pre- 

 dominate there, as might be expected. 



It is impossible to overlook the great similarity of these deposits of the Red 

 River valley, to those of the Tcche, of bayou Vermilion, C.'te Blanche, and even 

 Lake Charles. The peculiar red tint to which the river owes its name is almost 

 unknown in the Mississippi bottom above Red River, and eastward. On the other 

 hand, the rivers of Texas heading in the same geological region as upper Red River 

 (e.g. the Brazos and Colorado), seem to have cut their immediate valleys into 

 materials quite similar to those just described.' The curious alternation of Mis- 

 sissippi and Red River deposits in the alluvial banks of bayou Atchafalaya have 

 been repeatedly noticed by observers. 



The data thus far given clearly indicate as succeeding the Drift epoch, a period 

 of slow depression, diu'ing which the valleys of the larger rivers, already impressed 

 upon the surface previously, wore transformed into inlets reaching far into the 

 country; where the slow drainage of the continental waters, depositing their sus- 

 pended matter, gave rise to the formation of extensive swampy areas largely over- 

 grown with cypress, and traversed by meandering and frequently changing drainage 

 channels. As this depression gradually became more rapid, the paludal character 

 of the deposits changed into that of the silts, such as form the higher portion of 

 the Port Hudson profile ; and ultimately, the period of depression closed with the 

 deposition, inland, of the calcareous Loess or Bluff formation of the upper ]Missis- 

 sippi, and the overlying loams, now constituting the upland subsoils of the Gulf 

 States. 



We have here, beyond a doid)t, the counterpart of the " Champlain" depression 

 of Dana. But in order to establish full parallelism, it will be necessary to consider 

 the previous geological history of the Gulf of Mexico. 



I have in another paper- sketched the outlines of this liistory, from palteozoic 

 times, as recorded by the formations which, successively, have contributed toward 

 the filling up of the great INIississippi enibaymcnt, from Cairo southward. For the 

 purposes of the present communication, I need refer only to a few prominent points 

 discussed in that paper, viz: — 



1st. The existence of a cretaceous ridge, or axis of upheaval, marked by a series 



' Much has been said about the fertility of Red River valley as connected wilh the gypsum region 

 above. It is a remarkable fact, that the analyses made by Mr. R. II. Lougbridge, of Red River 

 soils and deposits from Louisiana, do not exhibit any unusual amount of sulphates ; nor does g\p.«um 

 occur in the clay beds. Probably the decomposing effect of the former upon the soil, through the 

 agency of atmospheric carbonate of ammonia, is the true cause of the great thriftiness. 



" Proc. Amer. Assoc. Adv. Sci., Indianapolis meeting, 1811; Am. Jour. Sci., Dec. 1871. 



