154 GEOLOGY OF SOME OF THE RIVER-SCENERY OF DERBYSHIRE. 



for manifestations of the same forces too striking to pass unnoticed, 

 and too stupendous in their results to be ignored. The Mississippi 

 carries into the sea 7,459,267,200 cubic feet of sediment every 

 year — a quantity sufficient to cover a square mile of ground to a 

 depth of 268 feet. The deltas of the Ganges, Amazon, and other 

 large rivers, are accumulations of sediment hundreds of feet deep 

 and thousands of square miles in extent, and have been formed 

 out of the mud, sand, and gravel worn away and transported from 

 the surrounding countries by the action of rain and streams. Yet 

 these vast accumulations represent but a portion of the waste 

 suffered by the land. Beyond the deltas, or estuaries, enormous 

 quantities also are deposited on the ocean floor. The turbid 

 waters of the Ganges reach far into the Bay of Bengal, and the 

 mud of the Amazon is observable for hundreds of miles out in 

 the Atlantic. 



It has been calculated that water running with a velocity of 6 

 inches per second will move fine sand ; with a velocity of 1 2 inches 

 per second will transport fine gravel, and with a velocity of 36 

 inches will sweep along angular stones of the size of a hen's egg. 

 The transporting and cutting power of rivers therefore depends 

 largely upon the rapidity of their currents ; it must not, however, 

 be supposed that running water of ' itselj "has much power to abrade 

 rocks ; the real strength of the denuding and cutting power of a 

 river lies in the sediment with which it is more or less charged. 

 Even when none is visible in the surface-waters, sand and gravel 

 are being hurried along the bottom, and their never-ceasing 

 friction wears away the bed, and is for ever deepening the 

 channel. So long as a river has a fall it will continue to deepen 

 its bed. To compare nature with art and to show the cutting 

 power of grains of sand, one may instance the method of cutting 

 large slabs of stone to be seen in a stoneyard. The instrument 

 used is a "saw-plate," or saw with a blunt edge. During the 

 sawing process, however, a constant stream of water and loose 

 sand is kept flowing below the saw-plate, and upon this the cutting 

 action practically depends. Again, to turn to ^Eolian agencies, it 

 is well known that in the western territories of the United States, 



