COOKE : JACKSON AND VICKSBURG DEPOSITS 189 



In the vicinity of Jackson the Yazoo clay probably does not 

 exceed 200 feet in thickness; at Shubuta it is reduced to 70 feet; 

 and in western Alabama it becomes of negligible thickness and 

 merges with the underlying member. The aggregate thickness 

 of the Jackson formation appears to be about 600 feet in western 

 Mississippi, about 230 feet at Jackson, and about 150 feet at 

 Shubuta. 



In Choctaw and Washington counties, Alabama, and in the 

 adjacent part of Mississippi, the stratigraphy of the Jackson is 

 somewhat different. The formation divides naturally into five 

 lithologic units, as follows: 



Subdivisions of the Jackson formation in ivestern Alabama and eastern Mississippi 



feet 



5. Yazoo clay member: Greenish gray calcareous cla}' with white cal- 

 careous concretions 8-50 



4. '"Zeuglodon bed.": Buff argillaceous marl with hard ledges. Tere- 

 bratulina lachryma, Aturia alabamensis,'^ Ostrea trigonalis, 0. falco, 

 Pecten perplanus, Schizaster armiger, Basilosaurtis cetoides 8-15 



3. Fine yellow sand with indurated lumps in the upper part. Well ex- 

 posed at Cocoa, Alabama 11-70 



2. Greenish yellow, calcareous, very plastic clay 30-50 



1. Hard yellow or brown impure limestone or indurated marl with Peri- 



archus lyeUi or P. pileus-sinensis and Pecten perplanus 0-15 



Division 1 of this generalized section appears to be largely 

 identical with the " Scutella bed" which Smith^ doubtfully 

 referred to the ''St. Stephens limestone." In a section at Willow 

 Branch^ I drew the Claiborne-Jackson line at the top of an 

 attenuated remnant of this bed, and Hopkins^ accepted this 

 correlation. My reasons for referring this bed to the Jackson 

 are the following: (1) Pecten perplanus and Periarchus pileus- 

 sinensis, species elsewhere restricted to deposits of Jackson age, 



^ I am probably to blame for the slip of the pen which caused Hopkins to list 

 Belosepia ungiila instead of Aturia alabamensis in the "Zeuglodon bed" (U. S. 

 Geological Survey Bull. 661-H: 296. 1917). Fortunately, I attached the cor- 

 rect name to the specimen figured by him on plate 27. 



^ Smith, E. A., Johx.sox, L. C., and Laxgdo.n', D. W., Report on the Geology 

 of the Coastal Plain of Alabama. Alabama Geol. Survey, p. 111. 1894. 



* Cooke, C. W., The age of the Ocala limestone. U. S. Geol. Survey Prof. 

 Paper 95: 115. 1915. 



^ Hopkins, O. B., Oil and gas possibilities of the Hatchetigbee anticline, Ala- 

 bama. U. S. Geol. Survey Bull. 661-H: 294, 297. 1917. 



