656 INDEX TO THE vSTRATIGRAPHY OF NORTH AMERICA. 



in Mississippi, although this will mean the expansion of the term as Hilgard used it, to include 

 the Tombigbee sand above, and its contraction with reference to the Mississippi representatives 

 of the Tuscaloosa formation below. 



The name Tombigbee sand, however, if applied to the actual type sections of the division 

 described by Hilgard and their equivalents, is expressive of a natural phase or subdivision of the 

 Eutaw formation and is eminently worthy of preservation in the literature. If we disregard 

 Hilgard' s imperfect mapping, the sections of the Tombigbee sand described by him are all 

 included within a thickness of strata which probably does not exceed 150 or 200 feet. Thus 

 linaited the division includes the uppermost massive, more or less calcareous and phosphatic 

 beds of the Eutaw formation, as distinguished from the more irregularly bedded and more 

 argillaceous portion of the formation beneath it. These upper, massive beds are traceable for 

 many miles to the north of Lowndes County in Mississippi and they extend eastward, with cer- 

 tain modifications, entirely across the State of Alabama into Georgia. They were recognized 

 by Smith and Johnson, who spoke of them as the ' 'upper member of the Eutaw formation." 

 This member, however, can. not be sharply differentiated from the remainder or lower member 

 of the formation, for massive lenses of glauconitic sand of greater or less extent occur in places at 

 lower levels than the Tombigbee member proper, and where these appear in small exposures 

 they 'are not distinguishable from that member. 



The Eutaw formation as thus defined (including the Tombigbee sand member) extends 

 from Alabama northward through Mississippi to the vicinity of the Tennessee State line, where it 

 ends against the unconformably subjacent Paleozoic rocks. 



The term "Rotten hmestone" was introduced by Alexander Winchell **'* in 1857. This 

 name was the commonly accepted designation of this great body of chalk rock until 1894, when 

 Smith, Johnson, and Langdon '^^'^ proposed the geographic appellation Sehna chalk, as a coname 

 with "Rotten limestone," and since that time the geographic term has been the accepted desig- 

 nation. It has been found that the Sehna chalk makes up all of the Upper Cretaceous strata 

 above the Tombigbee sand member in western Alabama and east-central Mississippi, where 

 it has an approximate tliickness of 1,000 feet, the terrane being overlain unconformably by 

 Eocene strata carrjdng characteristic fossilsfrom the eastern part of Marengo County, Ala., to 

 the northern part of Noxubee County, Miss. To the north in Mississippi and to the east in 

 Alabama the Sehna chalk as such becomes thinner and in each direction finally disappears. 

 The thinning of the formation is due, not to the actual tliinning or disappearance of the beds, 

 but to their replacement along the strike by nonchalky materials (sands, clays, marls, etc.) 

 having the same age and stratigraphic position. In the region of its fullest development the 

 formation is divisible on paleontologic grounds into two parts; the lower, embracing approxi- 

 mately the lower half of the formation, is most conspicuously characterized by the presence 

 of Exogyra foinderosa Roemer; the upper, embracing the remainder of the formation, is charac- 

 terized by the presence of Exogyra costata Say, and carries a fauna which corresponds in a 

 ceneral way with that of the Ripley formation of northern Mississippi, though it contains fewer 

 species. 



The Ripley formation, which in its type region in northern Mississippi conformably overlies 

 beds of the Selma chalk, is not younger than the youngest of the Selma chalk beds where the 

 latter are most completely developed in western Alabama and east-central Mississippi; on the 

 contrary, it is of the same age as the upper 300 or 400 feet of the Selma, the nonchalky, sandy, 

 and argillaceous beds of the Ripley being the representatives in northern Mississippi of the 

 upper part of the Selma chalk. To the north in Tennessee both the Selma chalk and the Ripley 

 eventually lose their distinguishing Hthologic characters, the former merging into marine 

 and estuarine equivalents and the latter into shallower-water equivalents, probably largely of 

 estuarine origin. The Ripley fornijation with these Hthologic modifications is traceable north- 

 ward through Kentucky to the head of the embayment region in southern Illinois, to which Hmit 

 the beds maintain their shallow-water characteristics. 



What has been said thus far applies more especially to the Cretaceous beds of the eastern 

 Gulf region between central Alabama and Mississippi River. The conditions of deposition which 



