THE 



GEOLOaiCAL MAGAZINE. 



No. XLVIII.— JUNE, 1868. 



I. — On Denudation now in Pkogress.^ 

 By Archibald Geikie, F.R.S., Director of the Geological Survey of Scotland. 



THE extent to which a country suffers denudation at the present 

 time is to be measured by the amount of mineral matter removed 

 from its surface and carried into the sea. An attentive examination 

 of this subject is calculated to throw some light on the vexed ques- 

 tion of the origin of valleys and also on the value of geological 

 time. Of the mineral substances received by the sea from the land, 

 one portion, and by far the larger, is brought down by streams, the 

 other is washed off by the waves of the sea itself. 



I. The material removed by streams is two-fold ; one part being 

 chemically dissolved in the water, the other mechanically suspended 

 or pushed along by the onward motion of the streams. The former, 

 though in large measure derived from underground sources, is like- 

 wise partly obtained from the surface. In some rivers the substances 

 held in solution amount to a considerable proportion. The Thames, 

 for example, carries to the sea every year about 450,000 tons of salts 

 invisibly dissolved in its waters. But the material in mechanical 

 suspension is of chief value in the present enquiry. The amount of 

 such material annually transported to the sea by some of the larger 

 rivers of the glol)e has been the subject of careful measurement and 

 calculation. Much has been written of the vastness of the yearly - 

 tribute of silt borne to the ocean by such streams as the Ganges and 

 Mississippi. But, as was first pointed out by Mr. Tylor, " the 

 mere consideration of the number of cubic feet of detritus annually 

 removed from any tract of land by its rivers does not produce so 

 striking an impression upon the mind as the statement of how much 

 the mean surface level of the district in question would be reduced 

 by such a removal."^ '\Vhen the annual discharge of sediment and the 



^ Abstract of part of a paper read before the Geological Society of Glasgow on 

 26th March, and which will appear in a forthcoming part of the Transactions of that 

 Society. 



2 Tylor, Phil. Mag., 4th series, v. 260 (1853). My attention was first called to 

 this very obvious and instructive method of representing the results of denudation 

 by some" remarks of Mr. Croll in the Phil. Mag. for February, 1867. Mr. Tylor's 

 earlier publication was afterwards pointed out to me by Professor Ramsay. Mr. Croll, 

 following up the line of argument suggested in his former paper, has gone into 

 further detail upon this subject in a memoir published in the Phil. Mag. for this 

 month (May), which will be of essential service to geology. 



VOL. V. — NO. XLVIII. 17 



