SHAPING THE EAETH — BOWIE 341 



to the streams and rivers. Another large part of the water that 

 falls to the earth soaks into the ground and absorbs a certain amount 

 of the mineral matter from the rocks. This water seeping through 

 the rocks will eventually reach streams and rivers and then will flow 

 to tidal waters carrying vast quantities of solid material with it. The 

 combination of the material in suspension and in solution results in 

 a large amount of continental matter that is transferred to sea areas 

 each year. 



It has been estimated that in the United States the rate of erosion 

 is approximately 1 foot in 9,000 years. Some areas, of course, have 

 very much more rapid rates of erosion than others, but this is the 

 average rate at which material is carried from the area of the United 

 States as a whole to tidal waters. The rate of erosion for the other 

 continental areas is probably just about the same as for our country. 

 This may not seem to be a very rapid rate, for during historic times 

 it would amount to only about one-half of a foot. The average ele- 

 vation of the United States is about 2,000 feet, and so to erode all 

 of the material lying above sea level would require something like 

 4,000 times the total length of the historic period. 



At this rate, however, something like 30 miles of erosion could 

 have occurred during the sedimentary age. Of course, there has been 

 no such amount of erosion as that. A particular exposed area that is 

 undergoing erosion is worn down to sea level eventually and then 

 erosion ceases, but it is rather remarkable that many areas which 

 have been eroded down to sea level have in a later period been raised 

 up again and thus other material has been subjected to erosion. 



It seems probable that the average elevation of the continental 

 areas has never been very much higher or lower than now. I believe 

 that if there has been any change, the average elevation has been 

 getting gradually lower. This is because the continental matter 

 carried to tidal waters is less dense than the subcrustal matter which 

 moves toward the continents to restore equilibrium. The average 

 elevation for all of the continental and island areas of the world is 

 slightly more than 2,000 feet, less than one-half mile, but there are 

 some parts of the earth where the elevations are 3 or 4 miles or 

 more. The maximum elevation of the Himalayan Mountains is more 

 than 29,000 feet, and there are mountain peaks in South America 

 and Alaska which are 20,000 feet or more in elevation. There are 

 great plateaus which stand more than 2 miles above sea level. But 

 these great elevations are offset by vast areas on continents and 

 islands which are only slightly above sea level. The ocean basins 

 have an average depth of approximately 10,000 feet. It seems rea- 

 sonably certain that some of these areas have changed their depths 

 during the sedimentary age. Some parts of the ocean bottoms have 



