FOSSILS THEIR NATURE AND INTERPRETATION. 79 



prising explorers ; or in the case of minute organisms buried 

 in the muds, the softer or destructible parts may decay 

 and pass away as gases or in solution. Generally, however, 

 fossils are but fragments or parts of the original structures 

 united during the life of the organism. Again, the origi- 

 nal substance of the fossil, when removed by solution after 

 fossilization, may be replaced by other mineral substance 

 brought in from without by infiltration. Or the mineral may 

 be molecularly changed or replaced ; an example is fossilized 

 wood, in which the grain and structure of wood is preserved 

 but silicified. This replacement may be by Silica, Calcite, 

 Pyrite, Marcasite, Siderite, and rarely some other minerals. 



Various Aspects of the Original Form represented. In the 

 fossil condition the form may differ from that we are accus- 

 tomed to see in the corresponding part of a living organism. 

 Thus a fossil snail-shell may be simply a fossil shell, that 

 is, the shell itself buried in the rock. Or it may consist of 

 the impression of the shell now removed, in which case it 

 may be the reverse or cavity over the exterior of the shell, or, 

 in case of flat shells, like clam-shells, similar impressions of the 

 inner surface ; or the cavity may be again filled with detrital 

 matter, forming a cast of either the inner or outer form of the 

 shell or object fossilized : in the former case it would be 

 called a mould ; in the latter, a cast. 



Preservation of Fossils. Fossils may have been covered un- 

 der various conditions and at various places; and the fossils 

 themselves are the best indication of the conditions. The 

 fossils may consist of land species alone, or types of organ- 

 isms adapted to live in air and not in water; but in order to 

 be preserved it is almost universally necessary that the part 

 fossilized be covered from the air : first, because atmospheric 

 conditions are extremely destructive to any substances exposed 

 to them, even quartz or glass suffering more or less by con- 

 tinuous exposure. The protection by soil will preserve the 

 more insoluble matters, but here again decomposition and 

 solution of any substance that can be decomposed or dissolved 

 will take place with slower or faster rapidity. Entire exclu- 

 sion from air and from circulation of acidulated and alkaline 

 waters is the condition under which the more perfect fossils 



