PRESERVATION OF FOSSILS. 27 



good artificial casts can be made. With the last two groups 

 the geological importance, compared with the other sections, 

 is usually small, and of course dependent directly upon the 

 degree of completeness with which the structural features are 

 capable of being inferred. 



The ever-shifting continental shore-lines are the lines of 

 sedimentation. On the existing sea-borders several general 

 zones of life can be readily made out. In the stratified rocks 

 of a geological province, the same geographical succession 

 of forms is capable of being determined with greater or less 

 distinctness. These ancient life zones correspond in a meas- 

 ure with the three general lithological features usually observ- 

 able in passing from an old coast border seaward: (1) the 

 coarse arenaceous deposits ; then (2) the argillaceous area, far- 

 ther outward; and finally (3) the limestones, extending into 

 what are known to have been deep-water tracts. In the broad 

 Mississippi province this arrangement of formations is well 

 shown, especially in the case of the later Paleozoic. 



The mode of preservation of the different fossils is there- 

 fore intimately dependent both upon the original character of 

 the hard parts, and upon the lithological nature of the deposits 

 In which the organic remains occur. In the first case, chemical 

 change is apt to take place more easily in organs containing 

 certain constituents than others. In the second instance, the 

 physical condition of the rock is an important factor. Thus, 

 certain molluscan shells containing a proportion of calcium 

 phosphate in addition to the carbonate, as in Lingula, are pre- 

 served, while associated shells composed of calcium carbonate 

 alone disappear entirely. Certain clay beds may be highly 

 charged with the remains of organisms, while a sandstone, 

 equally prolific of life originally, may allow the ready percola- 

 tion of subterraneous waters, dissolving and carrying away 

 rapidly the material composing the fossils. Also, many highly 

 fossiliferous limestones, in the common process of dolomiti- 

 zation usually have the original characters of the fossils con- 

 tained changed so that only the internal casts remain. 



