1864.] SILICIFICATION OF FOSSILS. 47 



masses of chalcedony a tenth of an inch or more in diameter. 

 This is an example of the second process, which is well illus- 

 trated by a fine specimen of a large and as yet undescribed 

 species of Metoptoma from the Birdseye formation, to which 

 my attention has been called by Mr. Billings. It was found 

 reposing on its base, and filled with the sedimentary limestone^ 

 which was removed by an acid, showing the interior of the 

 shell with some small adhering Serpulae, which are also silici- 

 fied. The exterior of the shell was completely covered with 

 a rough warty coating of chalcedony, which has evidently spread 

 in concentric circles from certain points, and is from five to ten 

 hundredths of an inch in thickness. This crust, which readily 

 separates, has been detached from a portion of the surface of the 

 shell ; which is found to have been completely replaced by chal- 

 cedony, and retains all its delicate markings. From the more 

 frequent absence of this exterior coating of chalcedony from 

 silicified fossils, we are inclined to look upon its deposition as a 

 process subsequent to the replacement. In some cases however it 

 takes place upon nou-silicified specimens. Thus a Stromatopora 

 having been cut in two, and submitted to the action of an a:id, it 

 was found that the silica was confined to an exterior crust, and to 

 occasional grains and portions disseminated through the calca- 

 reous mass of the fossil. It is further to be remarked, that the 

 limestone strata which contain the silicified fossils are associated 

 with beds or masses of hornstone, in which these fossils are some- 

 times partially imbedded. 



The facts detailed above (a part of which will be found in the 

 Geology of Canada, p. 629) point to the conclusion that the re- 

 placement of the fossils, as well as their incrustation and filling-up 

 with silica, took place before they were imbedded in the calcareous 

 sediments, and that it was dependent on the presence of silica 

 dissolved in the waters of the time. The mode in w hich the first 

 process, or that of replacement, has been efi'ected is however still 

 obscure. In vegetable structures, which are very often silicified, 

 such a replacement is comparatively rare. The pores of the wood 

 become filled with silicious matter, while the woody fibre, in a more 

 or less altered state, remains, and may be extracted, as Goeppert 

 has shown, by dissolving the silica with hydrofluoric acid. This 

 organic matter is often changed into coal, or even, according to 

 Dr. Dawson, in some Devonian woods into a graphitic substance ; 



