DYNAMICAL GEOLOGY. 211 



deepen its channel either proportionately or more rapidly, so 

 that it was never diverted from its former course. 



Independently of Medlicott and Powell, Tietze arrived at 

 a similar explanation of the origin of transverse valleys in the 

 Elburz mountains in Persia, and of the Iron Gates of the Danube 

 across the Transylvanian mountains. Tietze refers the begin- 

 ning of such transverse valleys to a period when the chains 

 across which they pass had no existence as such, but still 

 formed part of a continental plain. The Swiss geologists, 

 Heim and Briickner, support this theory, but it has been op- 

 posed by Lowl, who accepts Riitimeyer's explanation that the 

 backward erosion of valleys may finally cut through watersheds 

 and even entirely through mountain-chains. 



Within the last few decades geographers have made great 

 advances in the detailed knowledge regarding the erosion of 

 river-channels, the diversion of river-courses, the serpentine 

 windings, the recession of watersheds, and the causes of 

 special forms of erosion such as river-terraces and pot-holes. 

 These are fully treated in Penck's Morphologic (vol. i., pp. 



2 59-3 8 5)- 



The first exact reports on the quality and kinds of material 

 transported by rivers were those made by Mr. Everest (1832), 

 who determined that the average annual amount of detritus 

 covered by the River Ganges amounts to T ^ by weight. 

 Under the auspices of the United States Government, a very 

 important series of investigations were carried out on the 

 Mississippi river. The accurate results obtained there by the 

 engineers Humphreys and Abbot showed that the proportion 

 of material held in suspension by the river was y*W by weight, 

 and that the total weight of earthy matter annually transported 

 to the Gulf of Mexico by the Mississippi river amounted to 

 812,500,000,000 pounds. (Report upon the Physics and 

 Hydraulics of the Mississippi, 1861.) 



Nearly all the great rivers have now undergone examination 

 in this respect, and the results obtained have given geologists 

 a much clearer conception of the actual rate of progress of 

 subaerial waste. In an able essay, entitled On Modern 

 Denudation, published in 1868, Sir Archibald Geikie made 

 careful calculations of the amount of material annually 

 transported by rivers, and showed how an irregular surface 

 can be entirely levelled to a plain by the subaerial agencies of 

 denudation. 



