I090 PRINCIPLES OF STRATIGRAPHY 



other Crustacea. This mold of the interior is commonly spoken 

 of as a "cast," which is wholly erroneous, since the cast reproduces 

 the original in a new substance, whereas the mold is a reverse 

 copy. Between the external and internal mold a cast may be formed 

 by infiltration of mineral matter or by artificial means. Not un- 

 commonly the removal of the shell by solution is followed by a 

 closing of the cavity between the external and internal mold, owing 

 to the pressure to which the enclosing rocks are constantly subjected. 

 In such cases, the more strongly marked surface features will be 

 impressed upon the smoother surface, or. in general, the features 

 of the exterior will be impressed on the mold of the interior, which 

 thus shows the normal external features, though weakened, to- 

 gether with a reversed impression of the interior. This, as shown 

 by J. B. Wood worth, is illustrated by many Palaeozoic mussels, 

 which are represented by internal molds {Stcinkerne). These show 

 the lines of growth and other features of the exterior of the shell, 

 and, on the same specimen, may be seen the mold of the scars mark- 

 ing the former attachment of the mussel. 



In the Tampa beds of Florida natural casts of corals occur. 

 The original corals have been removed by solution, but have left 

 behind hollow molds in which afterward geodes of chalcedony were 

 formed, the exterior of which accurately reproduces in silica 

 the form of the corals. On the whole, while natural molds — both 

 external and internal — are common, and characteristic of nearly 

 all porous rocks, natural casts are correspondingly rare. It should, 

 however, be noted that pseudomorphs are closely akin to casts as 

 here defined, since in them die replacement is pari passu with the 

 removal of the original substance of the shell or other hard struc- 

 ture, while in normal casts complete removal of the original sub- 

 stance precedes deposition of the new material. 



2. Tracks, Trails and Bnrrozvs of Animals. 



Tracks. These are made by vertebrates walking or hopping 

 along the soft sand or mud, which will register their footprints. 

 If the mud is very soft the footprint will be closed again by the 

 flowage of the mud. but. if it is viscous, or so nearly dry as to 

 remain permanent, the footprints may readily be preserved. Nu- 

 merous reptilian footprints are known from the Newark sandstones 

 of the Connecticut Valley and the related district of New Jersey. 

 Lull (i6) believes that these may have been partially hardened by 

 the heat of an underlving lava sheet, which was only recently 



