PAST AND PRESENT INTERPRETATIONS. 17 



Mississippi to the northern part of Noxubee County; this resulted in the production in Ten- 

 nessee and Mississippi of the nonchalky Ripley formation above the Selma chalk. Finally 

 there was another expansion of favorable conditions northward to a short distance north of 

 Houston in Chickasaw County, forming the long narrow strip of chalk rock which makes up 

 the extreme uppermost strata of the Cretaceous above the sands of the Ripley formation in 

 Oktibbeha, Clay, and Chickasaw counties. (See PI. IX.) 



In Mississippi, therefore, part of the Selma chalk is replaced by synchronous nonchalky 

 beds, and northward hi Tennessee all of the Selma is eventually thus replaced. In northern 

 Mississippi and in Tennessee the nonchalky time equivalents of the basal portion of the Selma 

 are referred to the Eutaw formation, and these equivalents include a part of the Tombigbee 

 sand member of the Eutaw formation and all of the Coffee sand member of the Eutaw. (See 

 Pis. IX, in pocket, and X, p. 20; also pp. 12-15 and 21.) The time equivalents of the upper 

 part of the Selma in Mississippi and northward in Tennessee, Kentucky, and southern Illinois 

 are referred to the Ripley formation. 



In eastern Alabama, immediately after the beginning of chalk deposition, the conditions 

 unfavorable to the formation of chalk gradually spread westward from Russell County, the 

 materials laid down in the east being nonchalky marine beds which are here referred to the 

 Ripley formation. Near the close of Upper Cretaceous sedimentation, as the record is pre- 

 served, these unfavorable conditions had reached nearly to the western part of Marengo 

 County, Ala. (See PI. IX.) There was then a sudden eastward extension of favorable condi- 

 tions, producing the long strip of chalk rock which extends above the sands of the Ripley 

 almost to the eastern part of Dallas County and which forms the extreme uppermost beds of 

 the Cretaceous along this distance. This is the "Prairie Bluff limestone" of Winchell. 1 As 

 shown by the fossils, this rock is exactly synchronous with the similar tongue of chalk in Oktib- 

 beha, Clay, and Chickasaw counties, Miss., described above. 



Paleontologic subdivisions. — The Selma chalk in the region of its fullest development is 

 divisible on paleontologic grounds into two parts; the lower, embracing approximately the 

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

 ponderosa Roemer; the upper, embracing the remainder of the formation, is characterized by 

 the presence of Exogyra costata Say and carries a fauna, especially near its top, which corre- 

 sponds in a general way with the fauna of the originally described Ripley formation exposed 

 in the bluffs of Owl Creek, 3 miles northeast of Ripley, Tippah County, Miss., though it 

 contains fewer species. 



RIPLEY FORMATION. 



Mississippi. — The typical materials of the Ripley formation consist of marine, more or 

 less calcareous and glauconitic sands, sandy clays, impure limestones, and marls. These 

 were originally described by Hilgard. 2 The bluffs of Owl Creek, 3 miles northeast of Ripley, 

 Tippah County, Miss., may be considered the type locality of the formation. One of the 

 classic Owl Creek exposures is shown in Plate VI, A. Eocene limestones (Midway formation) 

 which immediately overlie the Ripley formation hi the Owl Creek region were included by 

 Hilgard in the Ripley. This error in correlation was corrected by Harris, 3 in 1896. 



Tennessee and northward. — In the vicinity of the Tennessee State line and northward 

 in Tennessee all but the basal beds of the formation appear to merge along the strike into 

 shallow-water equivalents consisting of irregularly bedded, largely nonglauconitic sands and 

 subordinate clays, probably hi part of marine, hi part of estuarhie, and hi part of fresh-water 

 origin. Since the time of Safford 4 these have been correlated with the Ripley formation. 

 Then lithologic dissimilarity to the typical materials of the Ripley formation seems to justify 

 the use of a member name to designate them, and the name McNairy sand member, derived 



1 Winchell, Alexander, Statistics of some artesian wells of Alabama: Proe. Am. Assoc. Adv. Sci., vol. 10, pt. 2, 1S57, p. 90. 



2 Hilgard, E. W., Geology and agriculture of Mississippi, 1860, pp. 83-95. 



3 Harris, G. D., The Midway stage: Bull. Am. Paleontology, vol. 1, No. 4, 1896, pp. 22-25 (136-139). 



* Safford, J. M., On the Cretaceous and superior formations of west Tennessee: Am. Jour. Sci., vol. 37, 1864, pp. 360-372; Geology of Tennessee, 

 1869, p. 550, plates and map. 



105°— No. 81—14 2 



