340 



HISTORICAL GEOLOGY 



FIG. 339. Approximate distribution of land and sea in North America in 

 the middle of the Ordovician period. (Modified after Willis.) 



Sedimentation under varying conditions. In different 

 parts of this interior sea the conditions were not alike, and 

 hence the sediments are not the same in different localities. 

 Over the great central and western interior region limy ooze, 

 composed partly of the shells of animals, was the most impor- 

 tant sediment, and there we now find thick beds of limestone- 



