CONDITIONS OF FOSSILIZATION 281 



interior of known forms is, as has been previously said, to make 

 a study of molds and casts, and thus reduce the liability of mistakes 

 to a minimum. j 



THE LITHOLOGICAL CHARACTER OF FORMATIONS AS AFFECTING THE 

 PRESERVATION OF INVERTEBRATES 



The conversion of an organism into a fossil depends upon the 

 character of its skeletal parts, the material in which it is buried, 

 and the material brought in, in solution, by infiltration. The material 

 of which the skeletal part is composed varies in different groups, 

 being more durable in some than in others, and therefore plays an im- 

 portant part in the preservation 

 of the organism. The variation 

 in the lithological character of 

 the material in which the organ- 

 ism is buried also plays an im- 

 portant part in its preservation. 

 Certain organisms are preserved 

 as originals; others as molds 

 and casts, in the same forma- 

 tion and locahty. In this same 

 formation, but in a locahty of p-^^ i^.-Moid of the interior. 



different lithological character, 



those groups which were lost under the former condition may be 

 retained under the latter, and vice versa. 



Apparently a law could be formulated to the effect that organisms 

 of the same mineral composition will be preserved in the same man- 

 ner, as originals, molds, or casts. In reality, however, this is not 

 true. Organisms are more completely preserved as originals in 

 limestone; yet it is in limestone that we find the most casts by molecu- 

 lar replacement. Molds and casts are very common in sandstones. 

 As hmestone approaches dolomite, the molds and casts increase, 

 although we also find originals. We find molds in hematite; but 

 they are more rare than in sandstones. 



The most perfect fossils are found in sandy and clayey shales. 

 The Niagara group at Waldron, Ind., is made up of fine calcareous 

 shales which are overlain by limestone. In these shales we find 



