368 



noticed a single cell, of a spiral Poljtlialamian 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 coloring, 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 waslied 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 precisely analogous to those which produced de- 

 posits of the same material as long ago as the Silurian epoch. 

 In this connection, it is important to observe 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 

 accompanying the well-defined casts are of wholly unrecognizable 

 forms, having merely a rounded, cracked, lobed, or even coprolitic 

 appearance. Certainly many of these masses, which often compose 

 whole strata, were not formed either in the cavities of Polythal- 

 amia or Mollusks. The fact, however, being established beyond 

 a doubt, that Greensand does form casts in the cavities of various 

 organic bodies, there is a great probability that all the masses of 

 this substance, however irregular, were formed in connection with 

 organic bodies, and that the chemical changes accompanying the 

 decay of the organic matter have been essentially connected with 

 the deposits in the cavities, of green and red silicates of iron, 

 and of nearly pure silica. It is a curious fact in this connection, 

 that the siliceous organisms, such as the DiatomacetT, Polycis- 

 tinece, and Spongiolites which accompany the Polythalamia in 

 the Gulf Stream, do not appear to have any influence in the 

 formation of casts. 



The discovery by Prof. Ehrenberg, of the connection between 

 organic bodies and the formation of Greensand, is one of very 

 great interest, and is one of the many instances which he has 

 given to prove the extensive agency of the minutest beings in 

 producing geological changes. 



