INTRODUCTION 15 



of the overlying sediment will usually completely fill the interior 

 of the shell, and upon this filling is impressed the shell's internal 

 ornamentation (Fig. 3, D). The external and internal molds 

 have thus the exact form and surface characters of the exterior 

 and interior of the shell. (Good examples of these molds may 

 easily be made by taking a complete clam or ark shell, first 

 dipping it in water, and then filling and surrounding it with 

 plaster of paris moistened to a plastic state.) 



If the waters percolating through the earth's strata be rich 

 in some solvent, as carbonic acid, and poor in lime, they will 

 dissolve the calcareous shells from between their external and 

 internal molds, and carry them away as a solution of bicar- 

 bonate of lime, leaving hollows where were the shells. In such 

 a case one of three things happens : (i) there remains an open 

 cavity, as in much of the Devonian (Oriskany) sandstone of 

 eastern New York, Pennsylvania and New Jersey ; (2) through 

 the settling of the strata the external mold comes into contact 

 with the internal (Fig. 3, F), and the external ornamentation 

 thus becomes impressed upon the interior markings, as in many 

 brachiopod and pelecypod shells from the Paleozoic ; or (3) a 

 petrifaction wdll be formed ; i.e. through a change in the direc- 

 tion of flow due to a differential upheaval of the region, or some 

 other cause, the percolating waters become supersaturated 

 with, for example, silica, lime or iron ; these hollows will then be 

 filled and the foreign mineral will be shaped by the molds 

 already formed into a cast (Fig. 3, E). Such a cast will bear an 

 exact surface likeness to the original shell, but will lack entirely 

 its internal structure. It is in such formation of a cast that the 

 external and internal molds perform the true function of molds 

 as usually defined, since they are thus " that in which something 

 is molded or formed." (Casts of iron fossils must usually be 

 covered with a thin coating of melted paraffin to prevent a rapid 

 rusting (oxidation and hydration) upon exposure to the air.) 



When the shell has been removed, leaving an open cavity, 

 the shape and ornamentation of the original shell may be ob- 



