158 THE OCEAN 



know that the marks had once been made be- 

 tween tides. 



Although you cannot actually accomplish 

 this in the soft sand and mud yet you can 

 imagine how the successive layers would ap- 

 pear if they were so hard that you could pull 

 them apart to reveal the marks upon them. 



This is exactly what the scientists have done, 

 for through the countless ages since the world 

 began the mud and sand that once formed 

 beaches and flats has been hardened into stone, 

 and upon these rock pages of the sea's story 

 many wonderful things are inscribed. 



Each layer of sand, each stratum of mud, has 

 formed a distinct layer of rock, and oftentimes 

 these are easily separated and come apart in 

 broad sheets or slabs covered with marvellous 

 records of what took place upon the seashores 

 in the dim ages when the earth was young. 



Of course not every piece of sandstone 

 shows such marks, for vast areas of the ancient 

 shores were no doubt deserted and unmarred 

 by passing creatures. In other places storms 

 and waves washed the imprints from the flats 

 for days, months and years before a layer of 



