83 



On the Origin of Greensand, and its Formation in the 

 Oceans of the Present Epoch. By Professor J. W. 

 Bailey, West Point, New York. 



(Communicated by the Author.) 



As an introduction to the subject of this paper, it is proper 

 to refer to various observations which have been made of facts 

 intimately related to those which I wish to present. That 

 the calcareous shells of the Polythalamia are sometimes re- 

 placed by silica, appears to have been first noticed by Ehren- 

 berg, who, in a note translated by Mr. Weaver, and published 

 in the 'L. E. and D. Philosophical Journal' for 1841, vol. 

 wiii, p. 397, says : 



" I may here remark that my continued researches on the 

 Polythalamia of the Chalk, have convinced me that very fre- 

 quently in the earthy coating of flints, which is partly calca- 

 reous and partly siliceous, the original calcareous shelled 

 animal forms have exchanged their lime for silex without 

 undergoing any alteration in figure, so that while some are 

 readily dissolved by an acid, others remain insoluble ; but in 

 chalk itself, all similar forms are immediately dissolved.'' 



The first notice of casts of the cells and soft parts of the 

 Polythalamia was published by myself in the 'American Journal 

 of Science' for 1815, vol. xlviii, where I stated as follows : 



'' The specimens from Fort Washington presented me with 

 what I believe have never been before noticed, viz., distinct 

 casts of Polythalamia. That these minute and perishable 

 shells should, when destroyed by chemical changes, ever leave 

 behind them indestructible memorials of their existence, was 

 scarcely to be expected, yet these casts of Polythalamia are 

 abundant and easily to be recognised in some of the Eocene 

 Marls from Fort Washington." This notice was accompanied 

 bv figiu'es of well-defined casts of Polythalamia (1. c, pi. iv, 

 figs. 30, 31). 



Dr. Mantell also noticed the occui'rence of casts of Polythala- 

 mia and their soft parts, preserved in flint and chalk, and 

 communicated an account of them to the Royal Society of 

 London, in May, 1816. In this paper he speaks of the 

 chambers of Polythalamia as being frequently filled with chalk, 

 flint, and silicate of iron. ('Phil. Trans.,' 1846, p. 466.) To 

 Ehrenbcrg, however, appears to be due the credit of first 

 distinctly announcing the connection between the Polythala- 

 mia and the formation of Greensand, thus throwing the first 

 light upon the origin of a substance which has long been a 

 puzzle to geologists. In a notice given by this distinguished 



