154 EARTH-SCULPTURE BY WATERFALLS. 



No doubt, then, can remain of the fact that some rivers and running 

 waters excavate and alter their channels ; though the changed course 

 sometimes results from the deposition of sediment in the river-bed. 



Lyell has given a remarkable case of recent excavation in a bed 

 of modern lava of a channel from 50 to several hundred feet wide, 

 and 40 to 50 feet deep, by the river Simeto, flowing from Etna. 

 Scrope has also shown that similar phenomena have happened in the 

 volcanic region of Auvergne. In these cases the action of the river 

 has probably been excited by the flowing of a current of lava across 

 its course, so as to dam up the waters, and give them something of 

 the force of a cataract. 



Waterfalls, &c. The waterfalls and cataracts upon the line of a 

 stream afford some curious points of study. It is especially in these 

 cases that the increase of excavating power, derived by a river from 

 the solid matter which it transports, is most evident. 



Fig. 49. Balkan. 



A cataract is formed upon the river Eden, in Westmoreland, near 

 Kirkby Stephen, by some beds of calcareous red sandstone conglo- 

 merate. The pebbles which the river brings down, contribute with 

 the whirlings of the water to excavate many deep perpendicular 

 pits or potholes, similar on a small scale to swallow holes on the 

 mountain limestone ranges, or those romantic cavities on the Caldew 

 in Cumberland. Below many waterfalls in Wales and Scotland the 

 same effect is produced. Near Christiana are deep pits of this 

 kind, termed " Giants' Kettles," often from 30 to 40 or more feet 

 deep. They have a spiral form like a corkscrew, are about four times 

 as deep as wide, and contain the stones which were rotated to exca- 

 vate them. They occur in groups near the sea, often at a height of 

 1 200 feet; small kettles 18 inches deep have been excavated in eight 

 or nine years by small streams. * 



1 Brogger and Reusch, Quart. Jour. Geol. Soc., vol. xxx. p. 750. 



