100 



SOLUTION AND THE FORMATION OF MOLDS 

 AND CASTS. 



Carbonated, or otherwise acidulated waters will dissolve 

 calcareous fossils, il" the strata in which they are embedded 

 are pervious to the ^^'ater. Thus, a mold of the exterior of a 

 dissolved shell, for exam[)le, may remain in the rock, while a 

 mold of the interior, formed by the mud or sand which found 

 its way between the valves, will remain within the mold of 

 the exterior. If the rock is under pressure, it may happen 

 that the two molds are pressed ag'ainst each other, and the 

 stronger features of one may become supprinij)Osed upon the 

 weaker features of the other. Thus, fossil mussels may show 

 the external striae impressed on the internal mold, show- 

 ing' at the same time the muscular impressions in relief. 

 Occasionally the space between the two molds, i. e. that 

 formerly occupied by the shell, may be filled by infiltrations^ 

 and a cast of the original shell may thus be produced. 



MECHANICAL DEFORMATIONS. 



These are very common, and they are apt to give a false 

 impression of the form of the fossil. Distortion by vertical 

 compression is the characteristic method of deformation of 

 fossils in undisturbed strata. This pressure is due to the 

 shrinking of the strata on solidifying, and is especially 

 marked in shales. Most fossils have probably been affected 

 to some extent by such compression, and frequently the 

 resulting defornmtion is very marked. ^Vlien the fossils are 

 protected by a limestone concretion formed about them, 

 they probably remain unaffected by such pressure. In 

 laterally compressed beds the distortion may render the 

 fossil unrecognizable, while in beds in which cleavage is 

 developed, this may affect the fossil as much as the rock. 



All remains of animals and plants commonly undergo 

 more or less alteration after burial. The amount of alter- 

 ation which will occur during a given time varies in general 

 in inverse proportion to the relative amount of mineral 



