sir HtJGH MILLEE. 293 



one foot in twelve thousand nine hundred and seventy-eight 

 years, nearly half the rate at which the Mississippi erodes its basin 

 mechanically.*' It would probably be too much to suppose Scot- 

 land as undergoing as much chemical action relative to its size 

 as England, or, it may be added, Ireland, being much poorer 

 in calcareous rocks, but, taking this estimate of Mr. Reade's 

 as applying to the whole British Isles, we should have the estimate 

 of three million nine hundred thousand years — the most reliable 

 and moderate of those just given for mechanical action — reduced ^o 

 rather less than two million six hundred and sixty-seven thousand 

 years as the time in which these Isles, if equally denuded at the 

 present rate, and not affected by elevatory forces, would exist 

 as tracts lying awash in the sea.f 



In taking this method of rej^resenting the work of river-de- 

 nudation, geologists may seem to lay themselves open to the 

 charge of dealing with millions of years as if they were drops in 

 an ocean of time. But although we are certainly not justified 

 in drawing inimitably on past time, it is certain that ages long 

 enough to produce and reproduce these effects of denudation have 

 passed over the earth, and in dealing with a question of general 

 denudation, it seems the directest way to illustrate the compe- 

 tency of existing agents to bring about what the subject requires 

 me to postulate. 



Special proofs of river action it is not necessary for me to give. 

 Let the reader only, like the poet. 



Learn to wander 

 Adown some trottin' burn's meander, 



and in the undermined banks and raw scars he will learn sum- 

 mer lessons that the heavy and turbid rush of the winter torrent 



* It is pleasant to see how nearly coincident this estimate is with that made by Prof. 

 Prestwich for a restricted area^that of the Thames basin — in 1872. Solution, however 

 can scarcely be considered as actually lowering the surface of the country by the total 

 amount, as a proportion takes place underground. In limestone districts underground 

 action is perhaps in excess. 



t One of Mr. Reade's most interesting results, however, is the "curious way in which 

 the smallest percentages of solids in solution, such as are contained in water from the 

 granite and metamorphic rocks, rise in the aggregate, through the greater rainfall on 

 these, high lying, formations, to the total solids in solution from such specially soluble 

 rocks as constitute the Thames basin." 



