46 THE CANADIAN NATURALIST. [Feb, 



NOTES ON THE SILICIFICATION OF FOSSILS. 



By T. Sterry Hunt, M.A., F.R.S. 



Fossils replaced by silica are very abundant among the paleo- 

 zoic limestones of Canada. Some portions of the Corniferous lime- 

 stone are little more than layers of silicified shells and corals, with 

 a small amount of intermingled carbonate of lime ; and beautiful 

 examples of silicification are also found in various localities 

 throughout the limestones of the Trenton and Quebec groups. The 

 silicified fossils are confined to certain planes; unaltered calcareous 

 shells and corals being often found in the same limestone bed, 

 half an inch above or below a layer holding silicified fossils; and 

 even in these the replacement is sometimes confined to a portion of 

 the shell or coral. A careful study of a series of these silicified 

 specimens shows the operation of three distinct processes. First, 

 the replacement of the fossil, giving rise to an exact copy of it in 

 chalcedoaic quartz ; second, the incrusting by chalcedony of a 

 fossil thus replaced; and third, in some cases the filling up of the- 

 cavity of the replaced fossil, with chalcedony or with crystalline 

 quartz. The corals from the Corniferous limestone present ex- 

 amples of the first process, and are besides often filled or lined 

 with crystals of quartz. The same thing is to be seen in various 

 gasteropods from the Birdseye formation. Of these, the silicified 

 shells, from which the limestone has been removed by an acid, 

 preserve all their superficial markings ; but are often lined with 

 crystalline quartz, although at other times filled with the sedimen- 

 tary limestone. In two instances, where these shells had been 

 fractured, the fissure has been filled up with a tissue of chalcedony 

 identical with that replacing the shell. This chalcedony is gen- 

 erally found to have a botryoidal surface, and a concentric 

 structure, which however in some cases can only be discovered 

 by the aid of a glass. Specimens of orthoceratites from the same 

 formation show the exterior, as well as the septa and the siphuncle 

 beautifully replaced by silica. In some silicified gasteropods it is 

 seen, after removing the calcareous matter by an acid, that the 

 silicification is chiefly confined to the two walls of the shell, which 

 are completely replaced, while the middle portion remains calca- 

 reous, or is but partially penetrated by silica. The exterior of 

 these silicified shells is sometimes incrusted with mammillary 



