On the Silicification of Organic Bodies. 203 



stance of the animal, or the organic matter contained in the 

 shell, and that where such organic substance is not present 

 there, no silicifying takes place. In support of these views 

 of Von Buch, it may be remarked, that wherever shells are 

 found embedded in a silicious matrix, they are generally en- 

 tirely converted into silica, no doubt owing to the large quan- 

 tity of animal matter contained in them before silicification 

 took place. In no instance could I find a trace of calcareous 

 matter remaining, clearly showing that either a chemical 

 change had taken place, or a mechanical substitution of the 

 silica for the whole calcareous matter. 



In further proof of Yon Buch's theory, I may quote a paper 

 by Dr Bowerbank on a zoophyte {Alcyonites parasiticum) 

 which he found enclosed in an agate. The interest attached 

 to this paper is, that while we have no evidence of the time 

 required for the silicification of calcareous bodies, and of 

 woods and horny substances here, we have complete evidence 

 of the rapidity of the change from the slight amount of the de- 

 composition of so tender a zoophyte as an Alcyonium. On this 

 point Dr Bowerbank remarks, " Nearly the whole of the 

 animal within the agate is in a beautiful state of preservation, 

 but there are a few spots which present evidence of the com- 

 mencement of decomposition by the detachment of groups of 

 cells from the mass of the polypidom ; in these cases the re- 

 mains of the tentacles, as might be expected, are very rarely 

 to be seen, and the disrupted mass is totally without a sponge 

 fibre. The envelopment of a tooth, or of hard calcareous bodies, 

 such as shells, afford no definite information regarding the 

 time necessary to accomplish such an operation. The invest- 

 ment even of such bodies as the rigid endurable horny fibres 

 of that tribe of sponges which are usually to be observed em- 

 bedded in flint, cherts, and moss agates gives also a consider- 

 able range of time to accomplish the fossilisation ; but when 

 we see such a soft and perishable substance as the fleshy body 

 of the living Alcyonidce, and such delicate organs as the ten- 

 tacula of the polypes thus preserved with such evident appear- 

 ances of freshness and perfection, I own it excites in me the 

 greatest astonishment that there should have been so rapid a 

 deposit of silicious matter as must evidently have taken place 



