FOSSIL ANIMALS 



327 



by minerals deposited from the ground water, the mass thus formed 

 (called a cast) has the external shape of the original structure (Fig. 274). 

 Both molds and casts are fossils, though they include no part of any 

 living thing. 



Fig. 273. 



-Fossil chain coral, Halysites, found in Michigan. (From specimen in the 

 Museum of Geology, University of Michigan.) 



Mud in which tracks were made hardened as it dried, and was resistant 

 enough to keep its shape while new material was washed over it in the 

 next freshet. New and old deposits hardened into rock, and the two 

 slabs were readily separable at the level of the tracks. One slab bears 

 molds, the other casts (Fig. 275). 



SedirrCervtl 



Fig. 274. — Diagram illustrating molds and casts. Horizontal shading represents sedi- 

 mentary deposits, vertical shading the material subsequently filled in. a, mold of a shell 

 which has been dissolved away by ground water; b, cast formed by subsequent filling in of 

 the cavity of a; c, mold of a shell whose interior was filled with sediment; d, cast produced 

 by filling the mold represented in c. {From Schuchert, "Historical Geology," courtesy of John 

 Wiley & Sons, Inc.) 



Many bones and shells were dissolved away and replaced piecemeal. 

 That is, the most soluble parts were removed first and replaced by the 

 least soluble minerals which the ground water then carried. Less soluble 

 portions were removed later, and replaced by minerals then prevalent. 

 Different parts of the original bone are thus replaced by different minerals, 



