204 Proceedings of the Royal Physical Society. 
thus to entomb the animal in such a condition as proves that 
at the utmost but a few days must have elapsed before it was 
so far encrusted as to completely preserve the form and posi- 
tion of the animal, not by a sudden immersion in suppositi- 
tious silicious paste, impounding it instantly in its full 
vigour, but after by a slow and gradual decease ; for this 
condition which I have described of semi-protrusion of the 
tentacula is that with which every one acquainted with recent 
zoophytes in a living condition is so familiar as an indication 
of slow and undisturbed death by exhaustion. In this condi- 
tion of semi-protrusion of the tentacula I have seen the 
animals of Alcyonium digitatum, Alcyonidium parasiticum, 
Caryophyllacea Smithii, and numerous species of Sertularia 
and other zoophytes die, if allowed to do so without interfer- 
ence ; but if touched or disturbed, the tentacula are slowly 
withdrawn, and never again extended. It does not appear to 
me to be necessary, for the production of a fossil, that the 
whole of the silex should have been deposited immediately. 
We may readily imagine, that after the rapid deposition of the 
first portion induced by the full exposure of the animal matter, 
and the consequently strong elective attraction exerted by the 
animal for the earthy particles, that the remainder of the de- 
posit — the filling up of the interstices of the network — would be 
more slowly and regularly completed in accordance with the 
laws of crystallisation, as we find that from the surface of this 
animal there are the same series of crops of radiating c^lce- 
donic crystals that characterise the structure of the great mass 
of the moss agates which I have described in my paper on 
those bodies, published in the " Annals and Magazine of 
Natural History," vol. x. page 9. 
This prismatic semi-crystallisation, if we may reason from 
analogies afi'orded by the phenomena of crystallisation dis- 
played by salts formed by acids with earthy or metallic bases, 
is a rapid, and perhaps irregular operation, compared with the 
slow formation of the regular and well-defined crystals of the re- 
spective substances under consideration, and which crystals are 
probably produced without the interference of any other agent 
than that which is necessary for their own construction. The 
specimens of fossil wood now shown prove that during the 
