ORGANIC AGENCIES. 



115 



1. Molluscous Shells. These inhabit mostly shallow 

 water, and therefore accumulate mostly along shore-lines, 

 and may be observed by all who keep their mental eyes 

 open. Each generation takes lime from sea- water, and. 

 leaves it as shell on the bottom. These, therefore, accu- 

 mulate until deposits of enormous thickness and extent 

 are often formed. Sometimes the accumulated mass may 

 consist of one species, as in oyster-banks ; sometimes of 

 many species. The deposit may be purely shelly, or shell 

 mixed with mud, or mud with a few imbedded shells. 

 Again, on exposed shore-lines the shells will be broken or 

 even comminuted, and on quiet shore-lines, as in bays or 

 harbors, they will be perfect. These accumulations are 

 gradually hardened into limestone (Fig. 62). 



FIG. 02. Modern shell liinestoue. (After Scott.) 



Application. Now, all these different kinds of lime- 

 stone or shell rock are found far away from present seas 

 and high up in the mountains. We are thus often able to 

 trace out the shore-lines of previous geological times, and 

 determine not only the species which then lived, but also 

 the conditions under which they lived. 



2. Microscopic Shells. These are some of vege- 



