1901.] NATURAL SCIENCES OF PHILADELPHIA. 51o 



ON THE PROBABLE AGE OF THE ALABAMA WHITE LIMESTONE. 

 BY THOMAS L. CASEY. 



The Jackson stage of the marine Eocene may, and probably 

 does, offer several lithological characters in common with the Vicks- 

 burg, but in the nature of its fossils it differs so profoundly that 

 it is impossible to conceive of aught else than the lapse of a greatly 

 prolonged time interval between the two horizons. Among some 

 240 species of the Jackson group I collected at Moody's 

 Branch and at Montgomery, La., and the bluffs below, I am 

 unable to recognize more than eight or ten which are unmis- 

 takably identical with any of a still larger series which 1 have 

 found in the Vicksburg beds during a residence of nearly two 

 years. It is true that there are quite a number of Vicksburg 

 species so closely related to analogues of the Jacksonian as to con- 

 clusively indicate a direct descent from the latter, but many of 

 these species belong lo ibat class which, from the isolation of their 

 environment, are peculiarly slow in evolutionary changes, such, for 

 example, as Dentallum and Cadulus and some of the small 

 bivalves, which from their frailness must live very secluded lives. 



The fact which most distinctly proclaims the revolution of 

 environmental conditions that must have been brought about 

 during the interval in question, and the probably great lapse of 

 intervening time;, is that so mauy highly characteristic Eocene 

 forms, such as Venericardia planicosta, Verticordia eocense, Galyp- 

 traphorus, Pseudoliva, Capidus, Volutilithes, Papillina, Lapparia, 

 and other mollusks, besides a number of very characteristic Tur- 

 binolid corals, completely disappear and leave no descendant in 

 any way related to them, for there are no species occurring in 

 the Vicksburg strata which recall any of these forms. And 

 again, there are many distinct types in the Vicksburgian, such 

 as Tritonopsu and Lyria costata, the ancestry of which cannot 

 be satisfactorily traced from the Jackson, and which must have 

 required a long time for their evolution. In addition to these 

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