4 ON THE GEOLOGY OF LOWER LOUISIANA 



Tlie most obvious difference between the two pre files just referred to, is in the 

 quality and quantity of the material of the Grand Gulf age; the latter being 

 represented at Fort Adams, by a stratum of a hundred and seventy feet of a knotty 

 argillaceous sandstone. But at neither of the points mentioned, nor at any other 

 now known between Vickshurg and the mouths of the Mississippi, is tliere any outcrop 

 of a marine f OSS Hi ferous deposit above loio loater. 



The existence of a thin stratum of yellow calcareous loam beneath the Loess, at 

 this point, was the first instance of the kind with which I became acquainted. 

 According to observations since made by Dr. Little, this layer is the representative 

 of a stratum thirty feet thick, which, at Nevitt's bluff, two miles above Natchez, 

 intervenes between the Loess and the Orange Sand, or Stratified Drift ; being there 

 divided into fifteen feet of yellow loam with calcareous concretions, and the same 

 thickness of bluish-gray calcareous clay, beneath. 



At Ellis' Cliffs, twelve miles south of Natchez, the Orange Sand alone, according 

 to Dr. Little, forms a profile of seventy-six feet, capped by thirty feet of the Loess. 



My previous observations in the interior of Mississippi, in addition to those just 

 mentioned, rendered superfluous any farther examinations of the older formations 

 above Fort Adams, which lies on the State line. 



Below the latter point, the exposure of the Grand Gulf rocks, mentioned above, 

 continues for several miles, with apparently a gradual decrease of thickness and 

 elevation. It has been traced southward, by Dr. Little, as far as the crossing of 

 Thompson's Creek, on the Woodville and Clinton, Louisiana, road — about Lat. 

 31° 10'. 



The Stratified Drift proper is visible, near the river, as far south as Jackson, 

 Louisiana, but farther inland extends to a somewhat lower latitude. No trace of 

 it, or of the Grand Gulf rocks, appears at the next exposure on the Mississippi 

 Kiver, viz. at Port Hudson. 



As for the Loess, it appears fully developed and with all its characteristic features, 

 at Fort Adams, and for eight or ten miles below. Facing southward we perceive, 

 from the summit of the Blockhouse Hill at this place, a wilderness of the charac- 

 teristic sharp ridges, often foreshortened into veritable peaks, usually between three 

 svnd four hundred feet above the river, but sometimes as high at least as four 

 hundred and fifty. 



But, according to Dr. Little's observations, these phenomena become more and 

 more modified as we advance southward. The Loess deposit thins out, its materials 

 becoming poorer in lime and fossils, and assuming more and more the character of a 

 common fine-grained indurate silt or hardpan ; the transition being by insensible 

 degrees, while the two extremes are very obviously distinct. At the same time the 

 clayey substrata which, farther up stream, appear .ohly in patches, are here seen 

 more frequently and continuously until, at Port Hudson, they become predominant.' 



' A detailed description of this region of transition will be given by Dr. Little hereafter. 



