22 THE OCEAN RIVER 



which the Ocean River has followed during its life. Fortu- 

 nately, the River bed and its boundaries have left tell-tale 

 evidence for the scientist to unravel. 



During the two billion years or so which have gone by since 

 the earth's crust solidified, the underlying basalt cooled fur- 

 ther and a continuous shrinking of the outer skin took place. 

 As a result of this, some geologists believe that the granite skin 

 has been wrinkled up, on as many as ten or twelve different 

 occasions during the history of the earth, into huge mountain 

 chains. During this mountain building, local weakening of the 

 earth's crust brought about periods of great volcanic activity, 

 that poured forth enormous volumes of lava. At the climax of 

 such a period the highlands must often have been covered by 

 enormous glaciers. 



After each period of mountain building there was a corre- 

 sponding and far longer period of mountain decay, according 

 to the older theories. The effects of rain and wind and of alter- 

 nating heat and cold were to dissolve and break up the rock 

 into boulders. These became still further reduced in size when 

 they fell into streams and were ground together until they 

 washed down the mountain slopes as gravel and sand. Eventu- 

 ally in the lowlands they became mud. Meanwhile the weather- 

 ing processes greatly reduced the size of the mountains. 



Where the rivers emptied into the shallow parts of the seas 

 bordering the continents they deposited their load of mud and 

 sand to form layers of sediment. Even today, millions of years 

 after the last great mountain growth, the rocks of the United 

 States alone are losing by this kind of erosion 800 million tons 

 every year. The results of erosive action have been to deposit 

 layers of sediment up to a thickness of thousands of feet in a 

 single period between the times of mountain building. Each 

 inch of this layer represents an average of two centuries of 

 time. 



As the mountains disappeared, the accumulated sediments 



