214 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



Under Buckland's term of "denudation," geology at the 

 present day signifies that process which, if continued far enough, 

 would reduce all surface irregularities of the globe to a uniform 

 base-level, but the general term makes no premisses about the 

 particular agencies affecting the removal of surface material. 

 The chief qualifying terms in common use at the present time 

 are " subaerial," " marine," and " submarine." Subaerial denu- 

 dation practically comprises all the natural operations by which 

 land-areas can be lowered ; it includes the action of wind, of 

 running water, and of ice. Marine denudation, so far as it 

 affects land-areas, is limited to a narrow marginal belt. Sub- 

 marine denudation is used to signify the wearing or scouring 

 action of the water, or any chemical processes affecting the 

 floor of the ocean. 



Hand in hand with the advance of scientific thought regard- 

 ing the causes and effects of recent denudation, there developed 

 among geologists a clearer apprehension of the evidences of 

 denudation in the past. In the beginning of the nineteenth 

 century, Berzelius and Hisinger had suggested that the sedi- 

 mentary series (Silurian) present in West Gothland might be 

 only remnants of a much wider sheet of deposit which had 

 been for the most part washed away. An important step in 

 advance was made by Sir Andrew Ramsay in his work On the 

 Denudation of South Wales (1846). Ramsay showed that the 

 Palaeozoic sedimentary .strata of Cornwall and South Wales 

 were composed of fragments derived from older rock-material, 

 that therefore this district had suffered immense loss by denuda- 

 tion in very early geological epochs. 



Emmrich in 1873 had drawn attention to the evidences of 

 transportation of Triassic rocks in Southern Thuringia, and in 

 1880 Biicking made an approximate estimate of the amount of 

 denudation, calculated from the thickness and extent of the 

 derived deposits. The researches of Pomel and Zittel in the 

 Libyan Desert and the Algerian Sahara, with their numerous 

 isolated hills, proved that this area had been denuded on a 

 scale of remarkable magnitude, probably by subaerial agencies 

 during the Pliocene and Diluvial periods. Dutton's famous 

 work on the Grand Canon showed that the extensive denuda- 

 tion of the Colorado lands had been likewise accomplished 

 within comparatively recent geological epochs. 



Neumayr, who made in 1885 a special investigation of the 

 original distribution and extent of the Jurassic formation, 



