OBLITERATED COIS'TINENTS. 149 



invasion of old age upon the beauty, the symmetry and 

 the habitability of continents, by raising the question of 

 the rate of erosion of their surfaces. If we look about 

 us, we discover the evidences of i?reat change in the con- 

 figuration of the hill-sides within a few years. One sum- 

 mer's rains plow unsightly gullies in our cultivated fields 

 and across our streets. These changes, resulting from 

 local transfers of earthy material, are filling lakes and 

 draining marshes, and transforming the hills ; but it is 

 only the transfer of the continental substance to the 

 ocean's bed which threatens the total obliteration of con- 

 tinents. The sediment carried down by rivers is an ex- 

 ponent of the efficient wastage, and the rate of disap- 

 pearance of the land. The sediments of the Mississippi 

 have been carefully measured by Humphreys and Abbott, 

 government engineers. The river discharges annually 

 sufficient earthy material to form a mass one mile square 

 and 268 feet deep. In other words, it is sufficient to 

 extend the bar at the mouth of the river 338 feet annu- 

 ally. They also estimate that the material of the entire 

 delta of the Mississippi may have been deposited within 

 5,000 years. These quantities of sediment are vast, and 

 impress us with a conviction that the solid land is disap- 

 pearing at a rate which is almost alarming. But these 

 volumes of sediment are gathered up from so vast an 

 area that the lowering of any particular square mile is 

 insignificant in any limited time. New York contributes 

 something to this deposit through the Alleghany and 

 Ohio rivers. The Rocky Mountains send their quota 

 to mingle with the mud floated from New York and 

 Pennsylvania; and all the great tributaries of this great 

 artery of the continent reach out their myriad fingers 



