COMPOSITION OF THE LOWER EOCENE. 595 



feet of calcareous sands and sandy limestone filled with marine forms, mainly 

 Turritella and Cardita. 



In the northwestern part of Butler county, Alabama, about 20 miles east 

 of the last-mentioned locality, this group becomes about 50 feet thick, the 

 pure Ncmtilus-hQi\.vmg limestone comprising the upper 30 feet, while the 

 sandier phase containing Turritella has increased its thickness to 20 feet. 

 From this point eastward the so-called Nautilus rock thins down, while the 

 Turritella phase increases both in thickness and in faunal variety until the 

 group is elevated from its insignificant place in the Alabama river section to 

 the most important group of the Eocene on the Chattahoochee river. On 

 this stream a section of this group is exposed, representing about 220 feet of 

 rocks, as follow^s : 



Section of Midwaii or Clayton Group ; Chattahoochee River. 



Feet. 



1 — "White calcareous sand, containing a few obscure casts and Ostrcea, sp. (?). 

 The sand sometimes becomes irregularly indurated, and is the source of 

 numbers of small lime springs. It forms the lowest part of the bluff at 

 Fort Gaines, Georgia, and in its uppermost 10 feet contains pockets of 

 vv'hite sand enclosed by black clay, the clay resting in "potholes " in the 

 limestone. Estimated at 200 



2 — Light-yellow, silicious limestone, containing a large Ostrcea and numerous 



obscure casts 10 



3 — Massive, coarse-grained sandstone, almost, a conglomerate 8 



Ripley group. 



The Black Bluff. — The identity of this group, in part at least, wuth Hil- 

 gard's Northern Lignitic is scarcely a matter of question, since the Alabama 

 survey has taken it up on the Succarnochee river where he left off. In Mis- 

 sissippi this author notes the occurrence of lignitiferous sands and clays 

 with leaf impressions to the extent of 400 to 500 feet, the marine life being 

 confined to a limited area at or near the base and through not more than 

 two feet of strata. The lack of lime in this group is clearly indicated by 

 the analysis of the derived soil, which shows less than fifteen-one-hundred ths 

 of one per cent. In the vicinity of the type locality on the Tombigbee river 

 the clays are but slightly calcareous, contain some marine forms, and are 

 about 100 feet thick. In Wilcox county, Alabama, near Allenton, some 50 

 miles east of Black blufl^, it is less than 65 feet. It contains a great variety 

 of faunal remains and is very calcareous, the black-clay feature being sub- 

 ordinated to thin ledges of limestone. On the Chattahoochee this group is 

 wanting entirely; in fact it has never been identified east of AVilcox county. 



The Naheola. — Next succeeding the Black Bluflf group in Smith's general 

 section is the Matthews' Landing or Naheola series, composed of gray sands 

 and sandy clays, containing at its base a fossiliferous stratum some 6 to 8 



LXXXVII— Bull. Geol. Soc. Am., Vol. 2, 1890. 



