On the Gnathodon beds around the head of Mobile Bay. 171 
nature, containing a smaller proportion of shells. 9. A thick 
mass of shells and sand. 10. At this stage, which approaches the 
level of tide water, a very marked change takes place in the fea- 
tures of the bed ; instead of shells, earthy matter now becomes pre- 
dominant, which here consists of yellowish sand, and other organic 
remains become less common. 11. Masses of red sandstone and 
conglomerate in a regular stratum, intermixed with sand and 
shells; dimension of the masses from the size of the fist to six 
inches in diameter. 12. Stratum of dark sandy marl, with a con- 
tinued deficiency of shells and other organic remains. 13. A mass 
of comminuted shells with a little earth and a few entire well pre- 
served shells. 14. The remainder of the section, extending only 
tion in this connection. It is about four feet in thickness, rests 
i nd 
upon a sandy base, and consists of a stratified mass o 
_ hation of the shell. 'Their decayed state, in the superficial exposed 
_ Part of a deposit, may give it all the appearance of an ancient 
: formation ; while those at the base, with the exception of the 
Mination of their antiquity very uncertain from the sole exami- 
