732 INDEX TO THE STRATIGEAPHY OF NOETH AMEEICA. 



The Midway group has been identified along Chattahoochee Eiver between Alabama and 

 Georgia, in the vicinity of Fort Gaines, whence it extends as a narrow belt, only a few miles 

 wide, in a northeast direction to Montezuma, on Flint Eiver, and a short distance beyond into 

 Houston County.'^ As the group in Georgia has not been subdivided into formations, it is 

 termed the Midway formation in this State. It is separated from the Upper Cretaceous by 

 small erosion unconformities, but at no place has there been found good evidence of any pro- 

 found physical break between the two. According to Veatch, "the formation is mainly a 

 marine deposit consisting of sands, clays, and marl or Umestone. The sand is varicolored, 

 generally loose or friable, and contains lenticular layers of white clay. A characteristic of the 

 lower part of the formation is a ra'ther wide distribution of limonite in thin crusts in the sand 

 and sandy clay and in the form of hollow concretions having black, polished botryoidal interiors. 

 The limestone is fossiliferous and in general highly arenaceous, but in a few places it is suffi- 

 ciently pure for use in the manufacture of lime. Friable marls, made up of glauconitic quartz 

 sand, clay, and shells occur, and also laminated black clay a,nd fuller's earth. The Hmestone 

 is conspicuous at several localities and is the phase of greatest stratigraphic importance because 

 it is abundantly fossUiferous, but sand and clay probably make up the greater part of the 

 deposit. The thickness of the Midway on Chattahoochee Eiver was estimated as 218 feet by 

 Langdon.* The thickness to the northeast is probably greater, and although it can not be 

 accurately estimated, it is probably as much as 400 feet." 



There are no known exposures of the Midway formation east of Houston County in Georgia, 

 nor are there any in South Carolina east of Santee Eiver. Sloan '"* has described an occurrence 

 in the vicinity of Georgetown, at Perkins Bluff on Black Eiver, of 16 feet of buhrrock, siliciiied 

 black shale, and compact red sands, immediately and unconform'ably overlying the Peedee 

 (Cretaceous). Species of Midway fossils were obtained from the material at this locality. 

 This is Sloan's Black Mingo formation, and he has also referred to it other outcrops along Black 

 and Sampit rivers. 



West of Alabama the Clayton limestone, the lowest formation of the Midway group, 

 extends into Mississippi, where it is represented by a series of hard crystalUne limestones and 

 calcareous sandy marls. The limestone of this formation was referred by Hilgard to the Eipley 

 (Cretaceous), but later investigation by Harris and others has, on paleontologic evidence, 

 placed it in the Midway. The limestone has a maximum thickness of 20 feet near the town 

 of Eipley. It is overlain by 20 to 30 feet of reddish to yellow sandy marl containing lime car- 

 bonate and is shghtly fossiliferous. The reddish color is due to a large amount of iron oxide. 

 The Clayton outcrop forms a narrow strip of territory from 2 to 6 miles wide, lying just west 

 of the Eipley (uppermost Cretaceous) area. The line of the New Orleans, Mobile & Chicago 

 EaUroad approximately follows the outcrop from Middleton, Tenn., to Houston, Miss., where 

 the outcrop turns in a southeasterly direction west of StarksvUle and Macon and passes into 

 Alabama southeast of Scooba." 



In Mississippi the calcareous sandy marls of the upper Clayton are overlain by 75 to 100 

 feet of gray nonfossUiferous clay, which forms the well-known "Flatwoods" area, extending from 

 Tennessee into Alabama. To this clay Crider applied the name Porters Creek clay, originally 

 used by Safford in Tennessee. It corresponds approximately to the Sucarnochee and Naheola, 

 formations of the Alabama section. 



The Midway group is represented in Tennessee by the Porters Creek formation as described 

 by Glenn.^''^ Harris presents m his monograph on the Midway, already cited, definite evidence 

 that the Porters Creek clay of Safford included Midway. According to Glenn, the Porters 

 Creek formation rests unconformably on the Eipley (Cretaceous). He says :'"''' 



" The Tertiary descriptions for Georgia are based on a manuscript furnished by Otto Veatch, who has since pub- 

 lished a report by himself and L. W. Stephenson.**^ . The paleontologic determinations and correlations were made 

 by T. W. Vaughan. 



6 Smith, E. A., Johnson, L. C, and Langdon, D. W., Report on the geology of the Coastal Plain of Alabama: Geol. 

 Survey Alabama, 1894, p. 369. 



c Descriptions of the Tertiary formations of Mississippi are modified from A. F. Crider's "Geology and mineral 

 resources of Mississippi " (Bull. U. S. Geol. Survey No. 283, 1906) and A. F. Crider and L. C. Johnson's ''Summary of 

 the underground water resources of Mississippi" (Water-Supply Paper XJ. S. Geol. Survey No. 159, 1906). 



