364 



Wyman, Dr. Samuel Cabot, Jr., Dr. N. B. Shurtleff, and 

 Mr. C. J. SpragLie. 



Prof. Wm. B. Rogers read, by its title, the following 

 paper " On the origin of Greensand, and its formation in 

 the oceans of the present epoch," by Prof. J. W. Bailey, 

 of West Point, N. Y. 



ON THE ORIGIN OF GREENSAND, AND ITS FORMATION IN THE 

 OCEANS OF THE PRESENT EPOCH, BY PROF. J. W. BAILEY. 



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 replaced by 

 silica, appears to have been first noticed by Ehrenberg, who, in a 

 note translated by Mr. Weaver, and published in the L. E. & D. 

 Philosophical Journal for 1841, (Vol. 18, p. 397,) says : — 



"I may here remark that my continued researches on the 

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

 quently in the earthy coating of flints, which is partly calcareous 

 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 1845, Vol. XLVIII., where I stated as follows: — 



" The specimens from Fort Washington presented me witii 

 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 recognized in some of the Eocene Marls from Fort 

 Washington." This notice was accompanied by figures of well- 

 defined casts of Polythalamia (1. c. pi. iv., fig. 30, 31). 



Dr. Mantell also noticed the occurrence of casts of Polythala- 

 mia and their soft parts, preserved in flint and chalk, and con\- 



