190 COOKE : JACKSON AND VICKSBURG DEPOSITS 



have been found in it at some places; (2) it is the first calcareous 

 bed of a series dominantly calcareous and succeeds noncalcareous 

 sands; (3) at Willow Branch and one or two other places there 

 is evidence strongly suggestive of unconformity between this 

 bed and the underlying Gosport sand. 



The other four divisions are the same as those described by 

 Hopkins^'* and need no further comment here. Sections illus- 

 trating them are given by Hopkins and Cooke in the papers 

 cited. 



The Jackson formation contains a large marine fauna. From 

 Jackson, Mississippi, I have listed 200 species of mollusks and 

 Vaughan has identified 12 species of corals; of these, about 49 

 are survivals from the Claiborne and about 15 are supposed to 

 have lived also in Vicksburg time. Canu and Bassler^^ list 

 67 species of Bryozoa from Jackson, of which 15 are known from 

 the Claiborne group and 9 from the Vicksburg. The commonest 

 and most significant vertebrate is Basilosaurus cetoides. 



OCALA LIMESTONE 



East of Tombigbee River a rather abrupt change is noticeable 

 in the stratigraphy of the deposits of Jackson age. The beds 

 become progressively more calcareous, lose their individuality, 

 and assume more and more the lithologic and faunal aspects 

 of the Ocala limestone of Florida. In some places a dual divi- 

 sion of these beds may be distinguished, but it is not everywhere 

 possible to draw a sharp line of demarkation between the upper 

 and the lower members. The lower part consists chiefly of very 

 argillaceous and somewhat glauconitic limestone, and on Sepulga 

 River the approximate position of the yellow sand at Cocoa 

 (division 3 of the generalized section) is occupied by calcareous 

 sandstone. The upper part, corresponding to the " Zeiiglodon 

 bed" and the Yazoo clay, consists of soft, cream-colored, amor- 

 phous limestone which closely resembles the ''chimney rock" of 

 the overlying Marianna limestone. 



10 Op. cit., 296. 



1' Canu, Ferdinand, and Bassler, R. S., Manuscript list of Eocene, and Oli- 

 gocene Cheilostome Bryozoa. 



