CONDITIONS OF FOSSILIZATION 



273 



E. The exterior of the skeleton may become coated with some 

 mineral such as silica, after which the skeleton may disappear. 

 In this case we should have a hollow mold. It would then be neces- 

 sary to take an impression, or cast, from this mold in order to ascer- 

 tain the external markings of the original, and by comparing this 

 cast with known forms we can determine its identity. At times 

 the corallites in a compound coral will become coated with silica, 

 and the spaces between the corallites filled with calcareous material 



Fig. 5. — Michelinia clappi. (After Hall.) 



the entire corallum having disappeared, leaving a mass filled with 

 these hollow siliceous tubes, the inner surfaces of which will be 

 molds of the exteriors of the corallites. (See Michelinia, pre- 

 viously referred to.) 



We may have the exterior coated with silica, the skeleton then 

 disappearing, and the space left filled with calcite. In a case of this 

 kind we have molds of the exterior, and it is impossible to identify 

 the fossil without first dissolving out the calcite, after which the 

 procedure will be as already described. 



