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, Alcyonidiurn 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 withdrawn, and never again extended.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 a,re the same series of crops of radiating calce- 

 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 afforded 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 



