COLLECTION AND PRESERVATION OF FOSSILS 347 



they may be found naturally produced through the 

 removal of the fossil in solution. Infillings of the 

 interior may also consist of the matrix, when that had 

 free access to the interior ; when it had no access, they 

 consist of some material deposited from solution, or 

 partly the one and partly the other, when the sediment 

 could only penetrate into part of the interior. Very 

 rarely does a hollow skeleton remain- really empty, 

 though infilling may often be incomplete. In limestones, 

 the material deposited from solution will be calcite, or 

 chalcedony (as in the chalk-flints) ; in clays it is usually 

 pyrite, or marcasite (the two forms of FeS 2 ). In cases 

 where deposit on the sea-floor was extremely slow, 

 calcium phosphate is a usual infilling, or for minute 

 cavities glauconite : these two substances being deposited 

 before burial is complete. 



In any of these cases it is only by subsequent removal 

 of the shell that the infilling appears as an actual 

 internal cast. Such removal may be by solution on the 

 sea-bottom or within the rock, or it may be artificially 

 produced, either accidentally during extraction, or de- 

 liberately for the purpose of investigating internal 

 structure. When naturally produced, the external and 

 internal casts together are often described as "hollow 

 casts": by pouring melted wax or gutta-percha, etc., 

 into the cavity, a mould of the fossil can be produced. 



5. There are other traces of living organisms in rocks 

 that may rank as fossils, though even less definite than 

 casts. Such are footprints and other impressions of 

 moving animals in sand or mud (preserved by being 

 covered by a lamina of a different kind of sediment), 

 burrows in sand or mud (preserved by being filled with 

 slightly different material) and borings in hard rocks or 

 in fossils. Among boring organisms are minute algae, 

 sponges (Cliona), some echinoids, many lamellibranchs 



