DYNAMICAL GEOLOGY. 2OQ 



advance of erosion he made a calculation of the age of the 

 Niagara Falls ; his result was afterwards modified by Wood- 

 ward and Gilbert. In the second half of this century, the 

 rate of river erosion has been examined in minute detail. 

 Physicists, geographers, and engineers have combined their 

 efforts to obtain an accurate determination of the rate of 

 movement in the different parts of a river-course, and the 

 corresponding capacity of the stream to transport solid 

 material. Geologists especially investigated the abrasive work 

 effected by the transported pebbles and sand in deepening 

 and widening a river-channel. In American literature, the 

 writings that had the most marked influence upon con- 

 temporary science were Dana's publications in the Reports of 

 Wilke? Exploring Expedition and his Manual of Geology 

 (1863), and Newberry's "Description of the Grand Canon ; ' 

 in his Report upon Colorado (1861). 



Oldham elucidated in 1859 the erosion of the valleys in 

 the Khasi Hills of India, Rubidge investigated the work of 

 water in eroding the South African valleys, and in 1870, 

 Blanford gave an account of the Abyssinian valleys. Green- 

 wood, Jukes, Whitaker, and Topley dealt exhaustively with 

 the erosion of English river-valleys. In 1869, Riitimeyer 

 published at Bale his famous work on Valley and Lake 

 Formation, which has exerted a permanent influence upon 

 geological thought. Riitimeyer endeavoured to prove that the 

 majority of the mountain-valleys in Switzerland, including the 

 largest river- valleys, had originated only in virtue of stream 

 erosion, but that long geological periods had been occupied 

 in the excavation of the channels. The commencement of 

 the valley erosion had been coeval with the uprise of the Alps, 

 but erosion had not always progressed with the same intensity. 

 Erosion had worked from the foot of the mountains backward 

 and upward to higher levels, consequently the different por- 

 tions of a river-course might present distinct types of erosion 

 (waterfalls, lakes, rivers, etc.). A sketch-map illustrating the 

 history of the lakes and rivers of Switzerland accompanied 

 Riitimeyer's work, and was of special value as the first bold 

 attempt to classify the Swiss valleys according to their geological 

 age. 



The American geologist Gilbert, in 1877, in his Geology of 

 the Henry Mountains, established the fundamental laws of river 

 action in the erosion of valleys. The researches of Powell and 



