i62 THE OCEAN 



stone and have not crawled about on the sea- 

 shores for millions of years. 



Many such fossils are found in the sandy 

 wastes of the Desert of Sahara, for the desert 

 was once the bed of an immense sea. In many 

 parts of the world vast deposits of this shell- 

 filled limestone occur and often where they 

 are far distant from the present level of the 

 sea. This proves that once the sea covered 

 these portions of the earth where the fossil 

 bottom is found and in addition it is often pos- 

 sible to determine whether the land rose and 

 caused the sea to recede gradually or rapidly. 



Very often these beds of dead shells and 

 other marine animals are hundreds of feet in 

 thickness, showing that the sea remained 

 above such places for ages and ages, while in 

 other localities the deposits are thin and 

 sparse, showing that they formed the bed of 

 the sea for a comparatively short time. In 

 some places they are smooth and in even lay- 

 ers, proving that the bed of the sea rose slowly 

 and steadily without disturbing the accumula- 

 tion at the bottom. But in other places the 

 layers of fossil shells are twisted and bent. 



