56 M. Von Buch on the Silicificatioii of Organic Bodies. 



tinous state, from the siliceous wart in their centre, can be proved 

 from their boundaries. They do not leave off where they meet 

 one another, or they do not go through each other as waves 

 would do ; but they are set off evenly at the side of one another, 

 each having a concentric series of rings, where there is no obstacle 

 to oppose their course. When the silicicating process is com- 

 pleted, the substance resists the storms of time, while the sur- 

 rounding chalk will be carried away, and the natural form of the 

 shell shows itself with all its peculiarities, more clearly than if it 

 had possessed its natural covering. 



Now, this is completely and universally the manner in which 

 bivalve shells become silicified. One will never fail to see 

 in them the siliceous warts with their surrounding rings. But 

 at the same time, we never see any system of rings spread it- 

 self over the shell, or extend itself within the substance of 

 the shell. In every case this last is raised up, while the sili- 

 ceous rings are found underneath. When these rings trace 

 out in their course the finest parts of the shell, so that we 

 can follow out the concentric lines and rays through all the in- 

 equalities of the calcedony, we must suppose that the calcedony 

 found another guide than the shell itself, when it received the im- 

 pressions of the shell during its onward course, for the shell 

 would have been a hinderance to the calcedony 's receiving these 

 impressions. This guide is the organic slime which is deposited 

 by the cloak upon the inside of the shell. 



I hope to be able to illustrate this most exactly, from the sili- 

 cifying process, as it is seen in Oysters. 



The oyster possesses, as we know, not only a very thick shell, 

 but the individual lamellae of this shell, which the animal forms 

 from within, gradually extend wider ; and are but very loosely 

 connected with one another. The irnier surface of the oyster 

 shell is covered over with the slime of the cloak of the animal, 

 and by that means becomes shining. This slime remains behind, 

 and will be covered with a new coating of shell, which will be 

 seen when we dissolve the shell in acid. The organic substance 

 is not dissolved. Likewise, it is easily seen by the unassisted 

 eye between the lamellae. Hence the oyster-shell consists of two 

 parts, one of which still belongs to zoology, the other to minera- 

 logy ; for the calcareous part is not carbonate of lime, in a zoo- 



5 



