M. Von Buch on tfie Silicification of' Organic Bodies. 55 



flint. New works upon fossils (of Sowerby, Conybcare, and 

 Brogniart) mention, that shells would be changed into a siliceous 

 substance, when they occurred in siliceous strata, but do not ex- 

 plain themselves in general, upon the disappearance of the cal- 

 careous shell. The case is not as here stated. The entire sili- 

 cifying process, as it can be easily traced in nature, leads to the 

 remarkable result, — That the silicifymg process never immediate' 

 ly attacks the calcareous shell ; that it developes itself only upon 

 tlie organic substance of the ariimaly and that where such an or- 

 ganic substance is not present^ there no siliciftjing takes place. 



When such a result is well established and proved, there na- 

 turally follows the important, and, in its application, the highly 

 fruitful position, that where the silicifying process is remarked, 

 there an organic substance must have previously existed. 



When a shell begins to be silicified, there appears over its 

 surface a small, dark coloured, semitransparent wart, probably 

 in a semifluid state like jelly. 



The white shell is raised up all around this small wart, whence 

 it follows that the wart has risen from within, and has not been 

 deposited from without. It spreads around, a new small wart rises 

 in the middle of the first, which now surrounds the new central 

 point like a small ring, and is separated from it by a deepened 

 space. Other small warts arise in succession, and push the ring 

 still farther back. And as this always takes place under the 

 raised-up calcareous shell, so this shell will be completely broken 

 and shivered by means of the siliceous rings, and the shell 

 falls oft* in small scales, and is lost. The rings become always 

 wider, but likewise constantly less elevated, until a new system 

 of rings comes in opposition, and each, as it becomes extended, is 

 the limit of the other. Thus one system of rings is added to an- 

 other, larger or smaller, according as they encounter each other 

 earlier or later, until the whole shell becomes silicicated. This 

 appearance is represented in Plate II. Fig. 2, as it is observed in 

 Gryphaea columba^ from Castellane in Provence. The half is 

 still covered with a thin calcareous shell ; but we see up<m the 

 edges plates which are separated by the rings. Some plates re- 

 main in the hollows between the rings ; and occasionally we 

 observe an entire piece of the shell, because no rings are found 

 under it. That these rings have spread out, when in the gela- 



