292 TYNEDAXE ESCAKPMENTS ; 



Croll, and then from better data by Professor Geikie, from whose 

 excellent memoir I slightly adapt the following.* 



It has been shown that the river Mississii)pi, whose drainage- 

 basin besides embracing a great extent of country, represents also 

 a variety of climate, brings to the sea annually an amount of 

 materials of which it is enough to say that they have been com- 

 puted as the equivalent of a compact block a square mile in extent, 

 and two hundred and sixty-eight feet high. Compared with some 

 others this great river seems to be a slow worker ; for while, at 

 this rate, it would spend six thousand years in lowering its basin 

 by one foot of solid rock equally distributed, the Po would take 

 only seven hundi'ed and twenty-nine years, and our own ^ith 

 only four thousand seven hundi-ed and twenty-three years to lower 

 their basins by the same amount. f The mean height of the 

 British Isles being about six hundred and fifty feet, the wear and 

 tear represented by the Mississippi, if taking place equally in up- 

 lands and lowlands, which of course it could not do, would level 

 them with the sea in three millions nine hundred thousand years ; 

 the Po, actually, if the estimate is to be trusted, woiking more 

 than nine times quicker, would, under the same conditions, level 

 them in four hundred and seventy-three thousand eight hundred 

 and fifty years; and the Nith would accomplish the same in 

 somewhat over three millions of years. 



Professor Geikie's data represent, as has been remarked, only 

 the mechanical action, not the chemical. I superadd an estimate of 

 the latter given by Mr. T. Mellard Keade, F.G.S., in an elaborate 

 ad(b*ess as President of the Geological Society of Liverpool. 



Mr. lleadc, founding mainly on the Sixth Report of the Rivers 

 Pollution Commission, calculates that an average of one hundred 

 and forty-three tons and a half of material is annually carried 

 away in solution from eveiy square mile of England and Wales ; 

 representing a general lowering from chemical action alone of 



• Tr«n*. Ocol. Soc. Glangow, Vol. III. Also Jukes tnd Gclkio's Mnnuol of Geology, 

 Chnp. XXV. 



t The entioutM for these rU'crx are nut to be placed at all on the same footing of re* 

 llaMllty, however, with that of the MiisUsippl. That for the Kith Is an estimate of solids 

 deposited In the Solwajr. and therefore i)fnores the silt carried out to sea by It* rapid 

 tldei.. 



