CONTINENTS; OCEANS 199 



This regular movement of the ocean surface is known as 

 the tide. If the shore is steep and precipitous like a wall, 

 the water literally rises and falls vertically, sometimes several 

 feet. Where the shore is gently sloping, with the rising of 

 the tide the water moves farther inland, covering a wide 

 beach, which is again uncovered as the tide falls. The 

 water mark upon a stake driven vertically into the sand 

 at the low-tide line will show to what vertical height the 

 water rises at high tide. There is an interval of a little 

 more than six hours between low tide and high tide. 



It is known that this piling up of the water is due to the 

 attraction which the sun and the moon have for the part of 

 the earth directly beneath them. The moon, because of 

 its nearness, has a stronger attraction than the sun. The 

 water, which is the movable part of the earth, is actually 

 lifted toward the moon, making a huge wave which follows 

 on across the ocean after the moon. 



Currents, waves, and tide have all had a share in 

 making the borders, and in some cases the interior, of 

 continents what they are now. 



220. The Life of the Ocean. The ocean has always 

 been of immense value to man as a source of food supply, 

 and its value is constantly increasing. Nor is its value 

 confined to the population living close to its shores. Thou- 

 sands of tons of salmon, sardines, and 'lobsters are annually 

 canned and distributed to places far from the ocean. Cod, 

 herring, and other fish are smoked or preserved by salt. 

 Newfoundland and Norway send supplies to warm coun- 

 tries where fish cannot be so well cured on account of the 

 heat. The use of ice and rapid transit make possible the 

 distribution of fresh ocean foods, and now more than ever 

 we depend upon the ocean for food. 



The living things of the ocean have by their shells and other 

 hard parts contributed greatly to making layers of rock, 

 sometimes thousands of feet thick. These rocks are now 



