738 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



banks of Chickasawhay River due west of the locality just mentioned, and at other places in 

 the same vicinity. They also outcrop at Garlandsville, Jasper County, and extensive beds of 

 earthy lignite underlie the marine fossiliferous Jackson at Jackson, Miss. At the latter place 

 "in the penitentiary well after passing through 32 feet of surface material and fossiliferous 

 strata of the Jackson age, lignitic clays were penetrated for 418 feet, after which a bed of 

 shells 20 feet thick, extremely rich in greensand, was passed through into water-bearing sand." 

 The lignitic sands and clays intervening between the marine fossUiferous Jackson and the 

 marine Claiborne occupy precisely the stratigraphic position of the Cockfield formation of 

 Louisiana, and the use of that name is extended so as to apply to them. 



Jackson growp. — This group, which is called a formation where not subdivided, has its 

 typical outcrop in the vicinity of Jackson, Miss., from which place it takes its name. 



In Mississippi, according to Crider and Johnson,"""" " the essential materials of the Jackson 

 group are gray calcareous and lignitic clays and sands. The outcrop occupies a belt of country 

 10 to 30 miles wide, extending southeast and northwest across the State from Yazoo to the 

 Alabama line north of Waynesboro. The area is known as the 'central prairie.' * * * 



"The Jackson has usually been described as 'marls' and clays, but recent investigations 

 along the line of contact with the Vicksburg have shown that there are between 50 and 75 feet 

 of yeUow, gray, or white siliceous sand at the top of the Jackson. Whether from a paleontologic 

 standpoint this should be considered Jackson or Vicksburg we are unable to say, since no fossils 

 have been found in the sands. They are regidarly stratified, showing that they were deposited 

 in a quiet sea with little or no current. In places near the surface the sands are slightly cemented 

 with iron oxide, causing some layers to resist erosion more than others." 



For Alabama Smith'""* says: "Above the Claiborne, and constituting the uppermost 

 member of the Eocene in Alabama, is the St. Stephens limestone, equivalent in part to the 

 Vicksburg Umestone and in part to the Jackson limestone of Mississippi. In Alabama these 

 two formations blend so completely that it has been impossible to draw clearly the line of 

 demarcation between them, and the St. Stephens is therefore intended to include the Alabama 

 representatives of both. Immediately overlying the Claiborne fossiliferous sands, at many 

 points in Clarke, Choctaw, and Washington counties, is an argillaceous limestone closely 

 resembling the Selma chalk and like it giving rise to rich black limy soils. The fossils of this 

 bed show that it is probably of Jackson age, but the great mass of the St. Stephens formation, 

 between 200 and 300 feet thick, consists of a limestone of a considerable degree of purity in 

 which the ever present fossil is a nummulitic shell, Orlitoides manteUi. Other shells also abound, 

 but this is characteristic." 



More detailed paleontologic investigation in Alabama wiU probably differentiate the 

 Jackson and Vicksburg. This opinion is borne out by some incomplete and unpubHshed 

 studies of Vaughan. 



Veatch states that "in Georgia the area underlain by the Jackson formation, so far as 

 known, is very small. The largest area is in Houston, Twiggs, and Pulaski counties. There 

 is an isolated occurrence at Rich HiU, Crawford County, one on Oconee River in Johnson 

 County, and probably one on Chattahoochee River near Alaga, Ala. 



" The formation is the uppermost Eocene exposed in Georgia and lies directly beneath the 

 Vicksburgian Oligocene. No evidence of a time interval between the deposition of the two 

 has been observed, and there seems to have been no interruption in the sedimentation, although 

 there are lithologic differences. Wtdle the formation is known to overlie the Claiborne, contacts 

 between the two formations have not been discovered. The Jackson at Rich HUl, Crawford 

 County, rests upon lower Cretaceous strata. 



"The Jackson consists mainly of white or cream-colored massive-bedded limestone, over- 

 lain by calcareous, glauconitic, or sandy laminated clay. The limestone is highly fossiliferous, 

 and in places the rock is made up almost entirely of a friable mass of Bryozoa and shells. 



"The thickness of the formation can not be determined with much accuracy, as the line 

 of contact between it and the Claiborne or underlying formation has not been definitely located. 



