COMPOSITION OF THE MIDDLE EOCEXE. 597 



Chattahoochee it thins down again to 44 feet, and is practically devoid of 

 organic life, only a few casts being noted. Some difficulty is encountered in 

 drawing the line between this and the underlying group because of the 

 absence of tlie lignite which serv^es Smith* as a line of demarkation. 



The Hatchetigbee. — The uppei*most member of the lower Tertiary, limited 

 by the Buhrstone on top and the Bashi at the bottom, is the Hatchetigbee. 

 It is composed, at the typical locality and in the region investigated by Smith, 

 of brown, purple, and gray laminated sandy clays and cross-bedded sands, 

 containing round concretions of indurated sand ; in all 175 feet thick. On 

 Pea river this series thins down to 25 feet, and on Chattahoochee river to 

 10 feet, the lithological features remaining constant. No fossils have been 

 found in this series except at Hatchetigbee bluff itself 



The Buhrstone. — The rocks of this series west of the Chattahoochee drain- 

 age consist of aluminous sandstones varying but slightly in composition and 

 meagre in quantity and variety of fossils. In the eastern part of Alabama, 

 however, the percentage of clay decreases, while the rocks become more 

 calcareous and the fossils more abundant. In lieu of the silicified casts 

 characterizing the Buhrstone on the Tombigbee and Alabama rivers, exten- 

 sive beds of Ostrcea seUceformis (var. divariccUa) and an Anomia occur. The 

 subterranean dissolution of the calcareous strata gives rise to numerous 

 limesinks in Henry county. 



On the Tombigbee the thickness of the series is estimated at 400 feet; on 

 the Alabama, at about 300 feet; and on the Chattahoochee, 190 feet. The 

 most noteworthy difference, however, is in the chemical composition of the 

 constituent rocks. 



The Clcuborne. — In the western part of the region under consideration 

 the Claiborne series is distinguishable into four sub-groups, viz., (a) the 

 Scutella bed, (5) the Ferruginous sands or Claiborne proper, (c) the Ostrcea 

 seUceformis bed, and (d) the Lisbon bed. 



The first subgroup is of minor importance, and serves mainly to mark 

 the boundary between the White Limestone and the Claiborne. 



The bed b has been more thoroughly studied by paleontologists than any 

 other of the southern Tertiary formations. It is, however, of very limited 

 extent, being confined exclusively to the region drained by the Tombigbee 

 and Alabama rivers. Thirteen miles west of Bladen Springs, Alabama, is 

 the westernmost outcrop of this stratum, which here consists of lignitic clays 

 containing some greensand and Crepidula lyrcda, Corbula cdabameiisis and 

 Voluta sayans. About five miles east of this point the bed consists of very 

 coarse-grained greensand with, but few fossils ; all, however, characteristic 

 of this horizon. Eastward of this last locality the stratum is normal in 

 appearance, except that the greensand is seldom decomposed as at the typ- 



* Bull. 43, U. S. Geol. Survey, 1886, p. 43. 



