102 



Shells of brachiopods and molluscs preserved by the infil- 

 tration of calcite are among the least altered remains, and 

 corals thus affected commonly retain all the details of their 

 structure. If the infiltrating mineral is silica, it will com- 

 monly lead to a complete silicifieation. 



2. Replacement. 



This occurs when another mineral takes the place of that 

 of which the skeleton originally consisted. It is commonly 

 a process of silicifieation, where silica replaces the original 

 substance, though calcification of originally siliceous struct- 

 ures is known to occur in sponges and Radiolaria (Zittel). 

 Silicifieation may conveniently be considered as occurring in 

 two ways. The first obtains when the decaying organic 

 tissue is directly replaced by silica, as in fossil wood. This 

 is a process of molecular substitution, where a molecule of 

 silica takes the place of a molecule of the wood, the inter- 

 change probably being due to some form of chemical re- 

 action. In such cases the microscopic structure of the 

 wood is usually retained in great perfection. The other 

 mode of silicifieation involves the replacement of the mineral 

 matter of the shell or other hard structure by silica, a pro- 

 cess which must be preceded by, or concurrent with, solution. 

 In this latter case the beginning of silicifieation is usually 

 marked by the appearance, on the surface of the fossil, of 

 a series of concentric rings (Beekite rings) surrounding a, 

 central elevation. This method of silicifieation commonly 

 destroys the microscopic structure of the organic remains, 

 but it gives them a mechanical and chemical stability, which 

 will insure the perfect preservation of the external forms, 

 and, furthermore, allow the fossil to weather out in relief. 



Fossils are also replaced by iron pyrites (or marcasite), by 

 iron oxide, and by other minerals. Such replacements 

 involve a chemical reaction between the replacing and origi- 

 nal substances. Pyritized fossils are among the most ex- 

 quisitely preserved organic remains, but they are subject to 

 disintegration on exposure. 



