NIAGARA 209 



ess. In one thick plate of glass a figure has been worked 

 out to a depth of fths of an inch. A second plate, Jths 

 of an inch thick, is entirely perforated. In a circular 

 plate of marble, nearly half an inch thick, open work of 

 most intricate and elaborate description has been exe- 

 cuted. It would probably take many days to perform this 

 work by any ordinary process; with the sand-blast it was 

 accomplished in an hour. So much for the strength of 

 the blast; its delicacy is illustrated by this beautiful ex- 

 ample of line engraving, etched on glass by means of the 

 blast. 



This power of erosion, so strikingly displayed when 

 sand is urged by air, renders us better able to conceive 

 its action when urged by water. The erosive power of 

 a river is vastly augmented by the solid matter carried 

 along with it. Sand or pebbles, caught in a river vor- 

 tex, can wear away the hardest rock; "potholes'* and 

 deep cylindrical shafts being thus produced. An ex- 

 traordinary instance of this kind of erosion is to be seen 

 in the Val Tournanche, above the village of this name. 

 The gorge at Handeck has been thus cut out. Such 

 waterfalls were once frequent in the valleys of Switzer- 

 land; for hardly any valley is without one or more trans- 

 verse barriers of resisting material, over which the river 

 flowing through the valley once fell as a cataract. Near 

 Pontresina, in the Engadin, there is such a case; a hard 

 gneiss being there worn away to form a gorge, through 

 which the river from the Morteratsch glacier rushes. The 

 barrier of the Kirchet above Meyringen is also a case in 

 point. Behind it was a lake, derived from the glacier of 

 the Aar, and over the barrier the lake poured its excess 

 of water. Here the rock, being limestone, was in part 



