Mineralogy and Geology. 



283 



6th. Abundance of organic casts, in Greensand, &c, of Polythalamia, 

 Tubuli, and of the cavities of Corals, were found in the specimen of yel- 

 lowish limestone, adhering to a specimen of Scutella Lyelli from the Eo- 

 cene of North Carolina. 



7th. Similar casts of Polythalamia, Tubuli, and of the cavities of Cor- 

 als, and spines of Echini, were found abundantly in a whitish limestone 

 adhering to a specimen of Ostrea sellseformis from the Eocene of South 

 Carolina. 



The last two specimens scarcely gave any indications of the presence 

 of Greensand before they were treated with dilute acid, but left an abun- 

 dant deposit of it when the calcareous portions were dissolved out. All 

 the above mentioned specimens, contained well-preserved and perfect 

 shells of Polythalamia. It appears from the above, that the occurrence 

 of well-defined organic casts, composed of Greensand, is by no means 

 rare in the fossil state. 



I come now to the main object of this paper, which is to announce that 

 the formation of precisely similar Greensand and other casts of Polythal- 

 amia, Mollusks, and Tubuli, is now going on in the deposits of the pres- 

 ent ocean. In an interesting report by Count F. Pourtales, upon some 

 specimens of soundings obtained by the U. S. Coast Survey in the explo- 

 ration of the Gulf Stream, (See Report of U. S. Coast Survey, for 1853, 

 Appendix, p. 83,) the sounding, from Lat. 31° 32', Long. 79° 35', depth 

 150 fathoms, is mentioned as "a mixture in about equal proportions of 

 Globigerina aud black sand, probably greensand, as it makes a green mark 

 when crushed on paper." Having examined the specimen alluded to by 

 Mr. Pourtales, besides many others from the Gulf Stream and Gulf of 

 Mexico, for which I arn indebted to Prof. A. D. Bache, the Superintend- 

 ent of the Coast Survey, I have found that not only is Greensand present 

 at the above locality, but at many others, both in the Gulf Stream and 

 Gulf of Mexico, and that this Greensand is often in the form of well-de- 

 fined casts of Polythalamia, minute Mollusks, and branching Tubuli, and 

 that the same variety of the petrifying material is found as in the fossil 

 casts, some being well-defined Greensand, others reddish, brownish, or al- 

 most white. In some cases I have noticed a single cell, of a spiral Poly- 

 thalamian cast, to be composed of Greensand, while all the others were 

 red or white, or vice versa. 



The species of Polythalamia whose casts are thus preserved, are easily 

 recognizable as identical with those whose perfectly preserved shells form 

 the chief part of the soundings. That these are of recent species is 

 proved by the facts that some of them still retain their brilliant red col- 

 oring, and that they leave distinct remains of their soft parts when treated 

 with dilute acids/ It is not to be supposed, therefore, that these casts are 

 of extinct species washed out of ancient submarine deposits. They are 

 now forming in the muds as they are deposited, and we have thus now 

 going on in the present seas, a formation of Greensand by processes pre- 

 cisely analogous to those which produced deposits of the same material 

 as long ago as the Silurian epoch. In this connection, it is important to ob- 

 serve that Ehrenberg's observations and my own, establish the fact that other 

 organic bodies than Polythalamia produce casts of Greensand, and it 

 should also be stated that many of the grains of Greensand accompany- 



