590 D. W. LANGDON, JR. CRETACEOUS AND TERTIARY STRATA. 



Feet. 

 2 — Red and gray variegated sandy clay; shows at water's edge, mouth of Bull 



creek, Georgia . 6 



3 — Massive white coarse-grained sand, held together by white clay 15 



4 — Hard clay stained by ferruginous matter; breaks with conchoidal fracture 1 



5 — Light green, highly micaceous sand resembling weathered schist, for which it 

 might be mistaken but for the occasional occurrence of water-worn quartz 

 pebbles 3 



Archean gneiss at Columbus, Georgia. 



Whether this thinning out is due to erosion subsequent to deposition, or to 

 less sedimentation originally, it is difficult to say. It is possible, however, 

 that the subsidence of the coast-line in this vicinity took place subsequent to 

 the deposition of the fossiliferous laminated gray clays occurring in the drain- 

 age of the Tuscaloosa and Cahaba rivers, so that only the younger or unfos- 

 siliferous member of this orroun is found on the Chattahoochee river. 



to" 



The Eutaw. — However slight may be the differences between the Eutaw 

 and the Tuscaloosa groups along the Tombigbee and Tuscaloosa rivers — 

 so slight indeed that Hilgard does not discriminate them, and Smith draws 

 the line with some misgivings because the mineral constituents and the 

 conditions of sedimentation are about the same, except that the highly col- 

 ored clays which are not found in the younger measures, — on the Chatta- 

 hoochee river, and for some distance westward, the fossiliferous char- 

 acter of the Eutaw deposits makes them easily distinguishable. 



Smith ^ has called attention to the absence of fossils in this group on the 

 Tombigbee and Tuscaloosa rivers, except in the uppermost part of the 

 measures. At Prattville, Alabama, which is near the base of this group, 

 numerous univalve casts are found. On the Chattahoochee the line between 

 the two groups is clearly marked by a bed of fossiliferous, laminated, sandy 

 clay containing Exogyra densata and an Anomia, which bed rests conform- 

 ably upon characteristic Tuscaloosa rocks. Scattered through the lower 

 ^art of this clay there are a number of lignitized logs filled with calcified 

 teredos, while in the upper part are numbers of casts as yet not identified 

 but consisting in the main of laraellibranchs. These clays appear to have 

 been deposited in undisturbed waters teeming with organic life, a condition 

 of affairs quite different from that existing synchronously in the more west- 

 ern waters, where in ever-shifting, possibly very cold, currents cross-bedded 

 sediments of sands, clays and pebbles, devoid of life, were deposited. Further 

 eastward, beginning near Prattville, conditions more favorable to the exist- 

 ence of marine mollusks seem to have obtained along this Eutaw coast. 



The thickness of the Eutaw is put by Smith f at 300 feet in his general 

 section ; and on the Chattahoochee it is practically the same, as follows : 



* Ihid., p. 88. 

 t Ibid., p. 18. 



