244 C. Lloyd Morgan — Physiography. 



it cuts its way downivards and cuts its way hackwards. Both, modes 

 of action go on, as a rule, at the same time ; but sometimes one, 

 sometimes the other, is most obvious. Of the first, the Canon 

 of the Colorado offers an example on the grandest scale. This 

 great ravine is about 300 miles long and, in places, more than a mile 

 deep. There can be no doubt that it has 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 grandest 

 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 under- 

 mined. It is in part owing to the undermining action, that visitors 

 can proceed 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 

 for a 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 Nature that, on 

 November 17, 1877, a large section of the rock towards the Canada 

 shore fell with a tremendous crash, and that during the night 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 

 watch any little rill which falls into a pool on the sea-shore, 

 we shall soon find out. We shall see that the sand and other 

 material which it carries are built up into a little delta, while some 

 of the finest material is spread at large over the bottom of the pool. 

 Large rivers carry vast quantities of mud and sand and silt (much 

 of which is washed off the land by the rain) to the sea. Experi- 

 ments of mine on the Thames at Surbiton show that in fine 

 weather, when the river was low and fairly clear, solid matter 

 in suspension was being carried seawards at the rate of 9767 

 tons per annum; while, when the river was in extreme flood, 

 matter at the rate of 524,940 tons per annum was passing 

 in this way down towards the sea. With the great rivers of the 

 world of course the amounts are still more enormous. Sir Charles 

 Lyell calculated " that if a fleet of more than 80 Indiamen, each 

 freighted with about 1,400 tons weight of mud, were to sail down 

 the Ganges every hour of every day and night for four months 

 continuously, they would only transport from the higher country to 



