1878. ] Physiography. 669 
great masses of the concave bank, the river having in this way ad- 
vanced upon the land hundreds of yards, and, in some cases, even, 
several miles, within the memory of living men. 
This shows how a stream cuts its way sideways into the land. 
This is not, however, the most important part of what a river 
does. If we follow our stream a little way inland, we shall discover 
that it cuts its way downwards and cuts it way dackwards. Both 
modes of action go on, asa rule, at the same time ; but some- 
times one, sometimes the other, is most obvious. Of the first, the 
Cañon of the Colorado offers an example on the grandest scale. 
This great ravine is about three hundred miles long and, in 
places, more than a mile deep. There can be no doubt that it ‘ha 
been entirely cut down into the desert plateau by the action of the 
river. How this was effected we learn, to some extent, from the 
following sentence in the American report on the river, “ The 
water of the Colorado,” says the reporter, “ holds in suspension 
a large amount of fine siliceous sand, sharp as emery, that eats 
away the valves” (connected with the machinery of the steamer) 
“as rapidly as it could be done with a file.” It has probably 
been with the aid of this sand that the river has cut down its deep 
trench. 
Of a river cutting its way backwards, the Niagara is the grand- 
est example. At the Falls the water tumbles over a ledge of lime- 
stone which rests on a thickness of shales. By the action of the 
spray which rises from the waterfall, and partly by the power of frost, 
the shale is rotted away, and thus the limestone is undermined. 
It is in part owing to the undermining action, that visitors can pro- 
ceed a little way under the Falls. To do so is well worth a wetting; 
a whole river takes its mighty leap, and falls with a bewildering 
roar at your very feet, and if it be winter giant icicles hang above 
your head. When the “ under-cutting” has gone on fora certain 
time, huge blocks of the limestone tumble with a crash to the base 
of the waterfall. In this way the Falls of Niagara are working 
backwards, at the rate of about one foot a year, towards Lake 
Erie. Only the other day it was stated in Mature that, on No- 
vember 17, 1877, a large section of the rock towards the Canada 
shore fell with a tremendous crash, and that during the Ee > a 
still larger area went down. 
But what becomes of all the material dug out by the stream as 
it cuts its way sideways, or downwards, or backwards? If we 
