322 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 3, 



or disturbed, the tentacula are slowly withdrawn and never again ex- 

 truded. 



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 imme- 

 diately. 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 deposit, the filling- 

 in of the interstices of the network, would be more slowly and regu- 

 larly completed in accordance with the laws of crystallization, as we 

 find that from the surface of this animal there are the same series of 

 crops of radiating calcedonic crystals that characterize 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 Na- 

 tural History,' vol. x. page 9. This prismatic semi-crystallization, if 

 we may reason from analogies afforded by the phsenomena of crystal- 

 lization displayed 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 respect- 

 ive 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. However that may be, it is 

 certainly quite a different operation from the merely mechanical de- 

 posit of the silex from the Geysers of Iceland or other such thermal 

 springs, the waters of which are charged with a considerable quantity 

 of the earth in solution. I have been favoured by Mr. C. C. Babington 

 with specimens of the silex deposited by the water of the Great Gey- 

 ser, which were procured by himself during a recent visit to Iceland, 

 and I have carefully examined and compared it with numerous speci- 

 mens of agates, cherts and flints, but this siliceous substance is not 

 like the latter bodies, formed in every instance more or less of fibrous 

 crystalline structure. Not a vestige of such an arrangement of its 

 particles could be discovered : it was purely amorphous, like a mass 

 of melted glass. In this form under some circumstances it is simply 

 a deposit arising from the evaporation of the water, and in other 

 cases it probably arises from the rapid disengagement of the excess 

 of carbonic acid in solution ; in either case it is most probable that 

 the silex thus solidified would present only the glacial form of that 

 deposited by the Geysers ; but if, on the contrary, the deposit be in- 

 duced by the modified and slow exertion of chemical affinity, then it is 

 probable that the silex would assume a form which approaches that 

 of its normal condition of crystallization ; and that this would be the 

 case is rendered most likely from the phsenomena which we observe 

 in the crystallization of nitrate of potass and other salts during slow 

 evaporation beneath the microscope assisted by the apparatus for the 

 polarization of light. 



If we view solutions of salts under these circumstances, we do not 

 observe the production of the crystals to be a slow and continuous 

 operation ; on the contrary, they are produced suddenly and at intervals. 

 We observe a single long prismatic crystal rapidly produced before 



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