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



say, since no critical examination of the fossils of the several divisions of the 

 Cretaceous has yet been undertaken. It is much to be regretted that the 

 divisions have been made on such arbitrary grounds as mere lithological 

 differences, since marked variations can be noted in almost any stratum of 

 any of the groups, and experience in both the Tertiary and the Cretaceous 

 of Alabama has proved the risk of creating groups on any but combined 

 physical and faunal differences. 



Noticeably absent from the Chattahoochee drainage is the probable Ala- 

 bama representative of Hilgard's Tombigbee sands,'!^ which marks so dis- 

 tinctly the boundary between the Rotten Limestone and Eutaw groups and 

 is the source of extensive deposits of phosphatic nodules. f This horizon has 

 not been traced east of Society Hill, Macon county, Alabama. 



The Ripley. — Immediately overlying the Eutaw group in the Chatta- 

 hoochee drainage is the Ripley group, characterized by gray calcareous 

 sands filled with large shells of Exogyra costata, Say, and Gryphcea mittahllls, 

 Mort. This group, reckoned at only 250 to 275 feet, with an outcrop barely 

 ten miles across on the Alabama river, becomes nearly 1,100 feet thick and 

 extends along the Chattahoochee river for thirty-five miles. Hilgard | esti- 

 mated its thickness in Mississippi at 350 feet. The estimate given below was 

 carefully made under favorable circumstances, and is believed to be cor- 

 rect. The relative importance of this group is much changed. Instead 

 of being the most insignificant group of the Cretaceous as in Mississippi and 

 western Alabama, in eastern Alabama and Georgia it becomes the most im- 

 portant in extent and economic value — i. e., as a soil-maker. On the 

 Alabama river, Avhere it has been studied to the best advantage, Smith § 

 notes only four well-defined horizons, as follows : 



Section of Ripley Group ; Alabama River. ^ 



Feet. 



1 — Yellowish micaceous sands (Ripley fossils) 55 



2 — Dark bluish-gray, sandy, micaceous clays, weathering into yellowish shales, 

 with indurated sandy ledges projecting at intervals of 5 to 10 feet through- 

 out ; whole thickness 100 



3 — Bluish argillaceous limestone, with phosphatized shell-casts, etc 30-35 



4 — Sands of various colors, dark blue to gray or white, traversed by indurated 

 bands of calcareous fossiliferous sands; contains many shells of Ostrcea 

 falcata 60 



Compare with this the great variations in the strata of this group along 

 the Georgia border : 



* Agric. and Geol. of Mi-s., 1860, p. 68. 



t Hull. 5, Dep. Agric. of Ala., 1884, and Bull. 43, U. S. Geol. Survey, 1887, pp. 85, 86. 



X Proc. Am. Assoc. Adv. Sci , vol. XX, 1871, p. '.^22; also Am. Jour. Sci., 3rd ser., vol. II, 1871, p. 391. 



g Bull. 43, U. S. Geol. Survey, 1886, pi. XXI, column 2, 



II Ibid., p. 179. 



