fcXCAVATION' O* 1 A LAVA CURRENT. 



201 



the character of the country through which the stream had flowed. 

 Below we give a diagram showing the excavation of a lava cur- 

 rent by the action of the river Simentb, one of the largest of the 



Sicilian rivers, which flows at the base of Etna. A A, bed of lava 

 which has flown to a distance of five or six miles; B, bed of the 

 Simento; C, foot of the cone of Etna; D, marine and volcanic 

 strata; E, ancient bed of the river. The lava current in which 

 the channel is eroded is one of the more recent, having been 

 ejected in 1603. In a little more than two centuries the Simento 

 has worn a passage from fifty, to several hundred feet wide, and 

 in some parts from forty to fifty feet deep. The portion of lava 

 cut through, is not porous, or mixed with cinders and scoria, but 

 consists of a compact homogeneous mass of hard blue rock. The 

 Falls of Niagara afford a magnificent example of the progressive 

 excavation of a deep valley in solid rock. It appears from exami- 

 nation that the Falls were once at Queenstown, about seve^i miles 

 below their present position. It is possible however that a natural 

 chasm may have previously been formed a part of this distance, 

 which the river has since widened, although a careful study of 

 the face of the country, and also the existing proofs, at various 

 places, several miles below the present falls, of fluvatile deposfts, 

 seem to show conclusively that the falls have gradually receded 

 from near the present site of Lewiston and Queenstown. 



When by the meTting of snows and ice, an unusual amount of 

 water is accumulated at some high point, and the barriers which 

 have been restraining it give way suddenly, the flood sweeps on- 

 ward with a fury which overcomes every obstacle. Such was the 

 flood in the valley of Bagnes, described by Mr. Lyell, in his Prin- 

 ciples of Geology. The bed of the river Dranee being blocked 



