STRATIFIED ROCKS. 203 



by solution, leaving an empty space corresponding to the 

 thickness of the shell. In d, this hollow space was sub- 

 sequently filled by deposit of soluble matters from perco- 

 lating waters. Cases c and d are represented by Figs. 

 119 and 120. 



Sometimes we have only the mold and cast of a small 

 part of an organism, as, for example, impressions of the 

 leaves of plants, or the footprints of animals walking on 

 the mud when it was soft. These, however, are of great 

 value, because they are very characteristic parts of plants 

 and animals. 



Finally, there are all grades of completeness of the 

 process of replacement. In bones, shells, and teeth, 

 sometimes only the organic matter is partly or wholly 

 replaced. Sometimes, also, the mineral matter is replaced 

 by other mineral matter. 



Distribution of Fossil Species. 



The kind of fossils which we find in the strata at any 

 place will depend on three things : 1. On the kind of 

 rock ; 2. On the country ; and, 3. On the age of the 

 rock. 



Kind of Rock. We have already said (page 130) that 

 at the present time different depths and bottoms are fre- 

 quented by different marine species. Some live on sand- 

 bottoms, some on mud-bottoms, and some on deep-sea 

 ooze. The same was true in previous epochs, and there- 

 fore we ought to expect and we do find that, in the same 

 country, and in strata of the same age, sandstones will 

 contain different fossils from limestones ; the one being 

 shore and the other open-sea deposit. Again, then as 

 now, lake-deposits contained fresh-water animals, and 

 estuary deposits land plants and animals ; and these are 

 of course different from marine species, though they be of 

 the same age and country. 



The Country. In rocks of the same age and same 



