On the Silicification of Organic Bodies. 203 
stance of tlie animal, or the organic matter contained in the 
shell, and that where such organic substance is not present 
there, no silicifjing 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 Von 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 Avith such evident appear- 
ances of freshness and perfection, I own it excites in me the 
2;reatest astonishment that there should have been so rapid a 
deposit of silicious matter as must evidently have taken place 
