24 ON THE GEOLOGY OF LOWER LOUISIANA 



Madre, is mentioned by Humphreys and Abbot ;^ but the additional statement that 

 it is there overhiid by. calcareous sandstone, a rock quite foreign to the formation 

 on the northern Gulf shore, made me hesitate about assigning them to the same 

 age. Mr. Loughridge has, however, found this sandstone near Lavaca, associated 

 with the characteristic clays ; the great predominance of lime in the deposits on 

 the Texan coast (abundantly exemplified by his collection) explaining the substi- 

 tution of calcareous sandstone for the loose sands which, farther north, are inter- 

 stratified with the clays. 



As for the occurrence of the characteristic materials of the formation in the more 

 immediate valley of the Mississippi and the Delta-plain proper, I cite the following 

 data. 



Blue clay containing sticks and leaves is found in wells in the Mississippi bottom 

 in the latitude of Greenwood, Miss., where it overlies water-bearing gravel, being 

 itself overlaid by " red" (t. e. orange-yellow) clay. Its thickness there is but a few 

 feet, perhaps on account of its proximity to the edge of the bottom. It crops out 

 in low places between McNutt and Sunflower River, and contains calcareous con- 

 cretions; it seems to be the material out of which the rich "Buckshot soil" is formed, 

 which soil is abundant also in the more northerly portions of the bottom. The 

 beds of the Yazoo, Sunflower, and Mississippi are themselves frequently cut into 

 this blue clay. At Bluck's mill, near the mouth of the Yazoo River, it has been 

 found forty feet thick (as at Port Hudson), and is underlaid by quicksand ; it con- 

 tains many sticks and leaves. 



The above data, taken from Himiphreys' and Abbot's Report, have been fully 

 confirmed by the late investigations of Dr. E. A. Smith, of the Mississippi Geo- 

 logical Survey.- Numerous characteristic profiles examined by him, on the Yazoo, 

 Sunflower, and their tributaries, clearly exhibit the non-fluviatile character of these 

 deposits, and their close correspondence, in every respect, with the Port Hudson 

 series. At the same time, the occurrence of the characteristic "buckshot soil" 

 resulting from their disintegration, renders their occurrence to within a few miles 

 of Memphis almost a matter of certainty. 



The existence of the blue clay opposite Vicksburg has been proven in digging the 

 canal ; but the materials of the Vicksburg hhiff are of a totally diff"erent character. 



At all the points just mentioned, it appears only at or near low-water level. But 

 at Nevitt's blufi" near Natchez (see above, p. 4), we have a profile corresponding 

 to that of the wells near Greenwood. IMoreovcr, Dr. Little has observed that the 

 beds of the larger streams below Vicksburg, as Bayou Pierre, Coles' Creek, and 

 Homochitto, are frequently cut into similar bluish or greenish clay, containing 

 vegetable remains, and not unfrequenfly Mastodon and other bones. These 

 deposits appear to underlie the Loess, and may be outliers or, rather, branches of 

 the main body of the deposit in the more immediate valley of the Mississippi. 



Opposite Natchez, according to Humphreys and Abbot, a thick stratum of blue 

 clay not only underlies the entire bottom, but between the Washita and Black bayou 



' Report on the Mississippi River, p. 101. 



' On the Geology of the Mississippi Bottom, Proc. Am. Ass'n Adv. Science, Indianapolis meet- 

 ing, 1871. 



