250 A, Geikie — On Denudation now in Progress, 



area of the river-basin are both known, the one sum divided by the 



other gives the fraction by which the area drained by the river has 



its general level reduced in one year. For it is clear that if a river 



carries so many millions of cubic feet of sediment every year into 



the sea, the area of country drained by it must have lost that quantity 



of solid material, and if we could restore the sediment so as to 



spread it over the basin, the layer so laid down would represent the 



fraction of a foot by which the basin had been lowered during a year. 



Thus the Ganges has its drainage area lowered by ^-^^ of a foot per annum.^ 



Mississippi „ „ ^^ „ 



Hoang Ho „ „ ^^ „ 



l^lione „ „ i:^ „ 



Danube „ „ ^g „ 



The laborious investigations of Messrs. Humphreys and Abbot '^ 

 have shewn that in the Avater of the Mississippi the amount of sedi- 

 ment is 1^0 by weight. This is a much smaller quantity than that in 

 many other rivers of the globe. Taking -it, however, as an approxi- 

 mate mean for the rivers of this country, we find that 



The Thames lowers its drainage basin by ~^^ of a foot per annum.^ 



It is much to be desired that careful measurements should be made 

 of the quantity of silt carried down by our British rivers. In the 

 case of the Nith a series of measurements and deductions made by 

 the engineer on the river led him to the conclusion that the quantity 

 of detritus borne by that stream into the Solway Firth reaches every 

 year an amount varying from 112,000 to 120,000 cubic yards.^' This 

 is equal to the lowering of the surface of the basin of drainage by 

 about —^ of a foot per annum. 



But besides the materials held in suspension there must also be 

 taken into account the quantity of sand and gravel pushed along the 

 bottom. In the case of the Mississippi this was estimated by the 

 United States Survey at 750,000,000 cubic feet. In our own rivers 

 it is probably on the whole proportionally greater. Indeed the 

 amount of coarse detritus carried down even by small streams is almost 

 incredible. Mr. Thomas Stevenson, the eminent harbour engineer, 

 informs me that at Lybster on the Caithness Coast, where a harbour 

 has been constructed at the mouth of a small stream, between 400 

 and 500 cubic yards of gravel and sand are every year carried down 

 by the stream. The area of drainage is estimated at about 4 square 

 miles. A weir or dam has been constructed to protect the harbour 

 from the inroad of the coarser sediment, and this is cleaned out regu- 

 larly every summer. A great deal of fine silt must be swept out to 

 sea, yet the portion of detritus caught by the weir, and annually 



1 These fractions represent the amount of solid rock removed from the drainage- 

 basins, allowing 1'9 as the specific gravity of the silt, and 2*5 as that of average rock. 



2 Ecport oil the Physics and Hydraulics of the Mississippi River. 1861. 



3 Appendix C to Second Keport of Tidal Harbour Commission, 1847, p. 603. 



