CONDITIONS OF FOSSILIZATION 



275 



cast of the interior, and has been regarded as having the structure 

 of the original organism. The conditions were evidently such as 

 to preclude its preservation in its original condition, or in such a 

 condition as to render its determination certain. 



authors' definitions of molds and casts 



Authors differ in their definitions of molds and casts. Some 

 make the terms synonymous. Others define them separately, 

 but are not consistent in 

 their application. 



Darwin^ considers the 

 mold as a matrix, and 

 the impression made by 

 an organism in this 

 matrix he terms a 

 "cast." 



Bernard^ applies the 

 term "mold" to three 

 distinct results: (a) to 

 an impression made by 

 the exterior or the in- 

 terior of a shell; {h) to 

 molecularly replaced 

 organisms; and (c) to 

 fillings of impressions. 



Gratacap^ applies 

 the term "cast" to (a) 



fiUingS which take the ^^^' ^' — MicheUnia dappi. (After Rominger.) 



place of organisms, to {h) the material filling the space occupied by 

 the soft parts. The term "mould" he appHes to impressions of 

 the exterior of an organism. 



White^ appHes the term "mold" to impressions made by the 

 organism. To the material filling this "mold" he apphes the 



1 Darwin, Geological Observations, Vol. II, p. 414. 



2 Bernard, op. cit. 



3 Gratacap, "Fossils and Fossilization, American Naturalist, Vols. XXX, XXXI, 

 pp. 288, 902, 903. 



4 White, Con. of Pres. of Invar. Fos., Bull. U. S. G. & G. S., Vol. V, No. i, p. 135. 



