170 A YEAR IN SCIENCE 



able in hilly and mountainous regions. The gullies 

 and ravines cut in hillsides attest this fact. The fan- 

 like deposits of soil often seen at the end of these gul- 

 lies and ravines also give evidence of the power of 

 rain in moving soil and earth. It has been estimated 

 that the Mississippi Valley is lowered one foot every 

 5,000 years by this erosion. As the Mississippi Valley 

 is considered to be an average one, this may be con- 

 sidered to be the average rate of erosion of the earth's 

 surface. Add to this the solvent effect of rain water 

 on the soil, and the total lowering is about one foot in 

 4,000 years. This impresses us with the fact that the 

 continents are indeed on their way to the sea. Tenny- 

 son in his "In Memoriam" has written: 



The hills are shadows and they flow 

 From form to form, and nothing stands : 

 They melt like mist, the solid lands, 

 Like clouds, they shape themselves and go. 



Questions 



1. Name and locate on a map of the world the five 

 great oceans. 



2. What proportion of the area of the earth's sur- 

 face do these oceans cover? What is their average 

 depth? 



3. Name and locate the continents. 



4. Do mountain, systems border oceans? Give 

 examples to illustrate your answer. 



