428 Miscellaneous. 



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-defined casts of Polythalaniia, 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 almost white. In some cases I have 

 noticed a single cell, of a sj)iral Polythalamian cast, to be composed 

 of greensand, while all the others were red or white, or vice versd. 



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 retaia 

 their brilliant red colouring, 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 precisely ana- 

 logous to those which produced deposits of the same material as long 

 ago as the Silurian epoch. In this connexion, it is important to 

 observe that Ehrenberg's observations and my own, establish the 

 fact that of/ier 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 un- 

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

 coprolitic appearance. Certainly many of these masses, which often 

 compose whole strata, were not tbrmed either in the cavities of Poly- 

 thalamia 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 connexion 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 connexion, that the siliceous organisms, 

 such as the Diatomacese, Polycistinese, and Spongiolites which accom- 

 pany the Polythalamia in the Gulf Stream, do not appear to have 

 any influence in the formation of casts. 



The discovery of Prof. Ehrenberg, of the connexion between 

 organic bodies and the formation of greensand, is 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 geo- 

 logical changes. — Proc. Bost. Soc. Nat. Hist. vol. v, p. 364. 



On the Cum.i;. By Prof. Agassiz. 



In a recent number of the ' Annals and Magazine of Natural His- 

 tory,' Mr. Bate describes some Crustacea related to Cumce, which 

 had young, and therefore ivere adults. This is not in conflict with 

 the statement of Prof. Agassiz in this Journal, vol. xiii. p. 420, 



