CHATTAHOOCHEE AND TOMBIGBEE STRATIGRAPHY CONTRASTED. 599 



Mississippi 215 feet, in western Alabama 200 feet, and on the Chattahoochee 

 275 feet. 



The uppermost or Salt Mountain division of this group is seen nowhere 

 east of the typical locality, and so does not enter into the geological features 

 of this section of Alabama. As a matter of fact, the characters of the Salt 

 Mountain limestone — i. e., corals and spines of echinoids — point rather to its 

 being an atoll built up in Tertiary seas than any extensive deposit justifying 

 the constitution of a group. 



The Miocene. — The strata that are referable beyond queston to the Mio- 

 cene are marine, and in Alabama and western Florida are confined practically 

 to the drainage of the Chattahoochee, though Mr. L. C. Johnson, of the 

 United States Geological Survey, has traced the Alum Bluff group as far 

 west as Yellow river. 



Resume. 



Probably the greatest difference between the geological features of eastern 

 and western Alabama occurs in the Cretaceous. 



The Tuscaloosa group of the Chattahoochee river is composed mainly of 

 sand and pebble beds ; no such accumulations are seen as on the Tuscaloosa 

 river and in Bibb and Chilton counties. Conspicuously absent are the leaf- 

 bearing clays, such as mark the lowest phase of the series further westward. 

 In thickness it has become reduced from a thousand feet to less than one 

 hundred. 



The Eutaw remains approximately constant in thickness, but in faunal 

 features a decided change takes place. From the Tombigbee river eastward 

 to the vicinity of Prattville, Alabama, no marine life has been noted, ex- 

 cept in what is termed the uppermost part of the measures, but what is 

 really either the Tombigbee sands of Hilgard or a lower member of the 

 Rotten limestone. From Prattville eastward molluscan remains increase 

 in quantity and in vertical distribution, being found in lessening number 

 and specific variety from the base of the measures to the top. 



The Rotten limestone thins out gradually eastward until it disappears 

 altogether on the Chattahoochee, causing the Ripley to rest directly and 

 apparently unconformably on the Eutaw. 



From the least important member of the Cretaceous formation in western 

 Alabama the Ripley has become dignified into the most extensive. 



The lower Tertiary, as a lignitiferous group, loses its identity in this re- 

 gion, since it is a noteworthy fact that there is not a single outcrop of lignite 

 on the Chattahoochee river, and the black and dark colored clays so charac- 

 teristic of this division of the Tertiary in western Alabama and Mississippi 

 have disappeared entirely. The strata of the division become more calcare- 

 ous, the faunal features more marine. 



