PALEONTOLOGY 



395 



If the deposition of the earth's layers have been laid down by water 

 and air, the various strata show such causes by forming a coarse sand- 

 layer, followed by a layer of finer sand or mud; or, two sandy layers 

 will be found separated by thin layers of muddy shale, the exact forma- 

 tion depending upon the velocity of both sand and water. 



Or, there may be mechanical and chemical changes which produce 

 beds of rock-sand or gypsum between beds of marl. Likewise organic 

 activities may have their influence as shown by the fine beds of coal 

 succeeding layers of sand, or by a layer of large fossils imbedded in lime- 

 stone. 



Fig. 245. 



