598 D. W. LANGDON, JR. — CRETACEOUS AND TERTIARY STRATA. 



ical outcrop on the Alabama river. No stratum that could be referred to 

 this horizon has been noted east of Monroe county, Alabama, or more than 

 twenty miles from Claiborne. 



Beneath the Claiborne sands lies a bed of gray calcareous sand charac- 

 terized by numerous shells of Ostrcea sellcejormis. Contrary to the opinion 

 of A. Winchell,* this has proved to be the most persistent member of this 

 group, having been traced by the writer personally from Suanlovey creek, 

 near Garlandsville, Newton county, Mississippi, to the Georgia line. In 

 general characters it does not vary throughout its extent, being about 75 

 feet thick on each of the three rivers. 



The lowest division of this group appears to be confined to the region 

 drained by the Alabama and Conecuh rivers. It is about 45 feet thick, and 

 differs from the preceding mainly in a peculiar group of fossils that have as 

 yet been but imperfectly studied. 



The total thickness of this group, as given in Smith's general section,t is 

 145 feet, as compared with 75 on the Chattahoochee, where the Ostrcea sellce- 

 jormis bed is the sole representative. 



The White Limestone. — The youngest member of the Eocene in the Gulf 

 embay raent was considered Cretaceous until Lyell's second visit to this 

 country, Morton \ having described the characteristic fossils and referred 

 them to the Cretaceous. To facilitate comparison, the group may be subdi- 

 vided into the Jackson and Vicksburg. 



Hilgard § mentions the occurrence of lignitic clay between the Jackson 

 and Vicksburg groups. Attention has already been called || to beds of lig- 

 nitic sand intercalated between the calcareous Jackson clays exposed on Pearl 

 river, Mississippi. At Red Bluff, Mississippi, near the Alabama line, the upper 

 part of the lignitic clays contains an abundance of marine forms, while at the 

 Alabama line these clays have become so calcareous and barren of fossils 

 that they blend imperceptibly into the prairie-making rocks of the Jackson. 



The Jackson sub-group, consisting of gray and white calcareous clays in a 

 general way devoid of fossils except a few Zeuglodon vertebrae, has been 

 traced from the Yazoo river to the Sepulgah, east of which it becomes more 

 sandy and ferruginous until, on reaching the Chattahoochee, it is almost a 

 counterpart of Smith's Scutella bed. 



The Vicksburg, or orbitoidal limestone is, at its typical locality, rather a 

 calcareous sand than a limestone, changing on Pearl river into occasional 

 ledges of indurated subcrystalline limestone, and from the Mississippi-Ala- 

 bama line to the Chattahoochee it assumes a white chalky phase which con- 

 stitutes practically the entire group. The total thickness of the group is, in 



* Proc. Am. Assoc. Adv. Sci., vol. X, part II, 1856, p. 86. 



tOp. cit., p. 18. 



X " Synopsis," 18.^3. 



i Agric. and Geol. Miss., 1860, pp. 107-110. 



II Author, Am. Journ. Sci.. 3d ser., 1886, vol. XXXI. 



