PKESENT CHANGES IN THE EAKTH. 99 



day, but this little daily work sums up largely when con- 

 tinued through centuries. Water does much erosive 

 work by rubbing one solid surface against another. This 

 we see on the sea-shore, as the waves, lashing the shore, 

 jostle the pebbles, great and small, against each other. 

 The same tiling is done with the grains of sand and mud 

 wherever there is water moving them. Every pebble 

 and grain that is carried down by a river toward the 

 sea grows continually smaller by being rubbed on its 

 passage by the accompanying grains and pebbles, as well 

 as by the friction of the water itself. We see this mode 

 of erosion exemplified in the pot-holes seen in rocks 

 where there is shallow water running over them. The 

 stones contained in them are rounded by the constant 

 friction, and at the same time wear the hole in the rock 

 continually larger. In the Franconia Notch of the White 

 Mountains there is a pot-hole in granite, called the "Ba- 

 sin," which is fifteen feet deep and about twenty in di- 

 ameter. 



183. Niagara Falls. One of the most striking exam- 

 ples of the erosive power of water we have in the Falls 

 of Niagara. The river runs over hard limestone, but 

 under this are soft shales. The result is that the water 

 at the Falls continually wears away the shales, and the 

 limestone falls from want of support below, the water 

 above pressing it downward. The edge of the fall is 

 therefore constantly receding at a rate calculated vari- 

 ously by different persons, from one foot to three feet 

 annually. This is a slow recession, but in the course of 

 centuries the change is a great one. There is the most 

 decided proof that the Falls were once seven miles far- 

 ther down the river than they are now, so that they 

 have receded all that distance, the process having begun 

 long ages before the creation of man. As the layers of 

 rock, instead of being perfectly horizontal, dip a little in 

 running back toward Lake Erie, the height of the fall is 

 constantly lessening as it recedes. It can therefore nev- 



