54 THE WORLD'S LUMBER ROOM. 



by the rivers Mora and Canadian. At the junction of the 

 two rivers the canon was at one time 860 feet deep, but 

 was afterwards filled to a depth of 470 feet by a stream of 

 basaltic lava from a neighbouring volcano. Since those days, 

 however, the united rivers have worked to such purpose 

 that the whole of the basalt and 230 feet more of the sand- 

 stone have been cut through, making the gorge now 1,090 

 feet deep. As a rule, rivers are more apt to deepen than to 

 widen their beds, the friction being greater at the bottom 

 than at the sides ; and usually the " weathering" of the cliffs 

 or banks, produced by frost, thaw, wind, rain, or air, goes on 

 more rapidly than the grinding below. But canon-making 

 rivers work at greater speed than these atmospheric in- 

 fluences, and the Canadian and Mora, after cutting a narrow 

 passage through the middle of the lava, went on to under- 

 mine it, so that it now projects in a wide terrace on either 

 side, deep down within the gorge. 



The neighbouring river of La Platte, on the other hand, 

 though its bed slopes on the whole as much as that of the 

 Colorado, and it has plenty of sand, makes no such canons, 

 for its load is just as much as it can carry, and more than it 

 can use with effect ; its banks, too, are not hard and solid 

 enough to stand as walls, and the loose sandstone of which 

 they are composed is blown or washed into the river as fast 

 as it crumbles down, and almost chokes the stream. Too 

 little sand, on the other hand, will be swept onwards without 

 making any impression, and it is when the quantity is nicely 

 proportioned to the force and volume of the current that 

 the most striking results are achieved. 



When pieces of rock are first broken off, whether by the 



