April 29, 1921] 



SCIENCE 



413 



away the country rock heneaih and heside it, 

 " thus cutting an ever-deepening trench " 

 (p. 37). 



3. Cleland* refers to the confusion in the 

 use of the terms corrasion, abrasion, corrosion, 

 erosion and denudation in a footnote on 

 pages 83 and 84, and then defines corrasion 

 and abrasion as synonymous and meaning 

 the detachment of rock particles as a result 

 of wear; corrosion as the work done by solu- 

 tion; erosion as the sum of corrasion and 

 corrosion, and denudation as a general term 

 for the lowering of the land surface by any 

 agency. 



i. Grabau,^ following Walther, defines ero- 

 sion as clastation plus ablation, or the break- 

 ing up of rock material and its removal. 

 Denudation is the removal of weathered and 

 loose mantle rock and is a subordinate part 

 of the process of ablation, which includes in 

 addition corrasion, quarrying and corrosion. 

 The former is defined as a " filing process " 

 which may be accomplished by wind, running 

 water, ice, waves or organisms, while corro- 

 sion is chemical removal by air (= evapora- 

 tion), water, heat or organisms. 



5. De Martcnne^ uses the term erosion in a 

 broad sense but does not define it. Corrasion 

 he limits to the wearing action of wind- 

 driven particles, as typically illustrated in 

 arid regions (pp. 660, 664). 



6. Kayser^ describes erosion as the mechan- 

 ical work of water, and denudation as the 

 sum of erosion and weathering, which latter 

 term includes both chemical and mechanical 

 action. Corrasion he defines in the same way 

 as de Martonne (p. 240), both authors here 

 following Walther and von Eichthofen. 



7. Park^ defines denudation as " the wear- 

 ing away, wasting, or breaking up of the 

 land surface, whereby the general level of the 



4 ' ' Geology, Physical and Historical, ' ' 1916. 



5 "Principles of Stratigraphy," 1913, pp. 17, 18. 



6 "Traite de GSographie Physique," Second 

 Edition, 1913. 



7 ' ' Allgemeine Geologie, ' ' third edition, 1909, 

 Vol. I., p. 260. 



8 "Textbook of Geology," 1914. 



land is lowered." Erosion, according to this 

 author, is embraced within the term denuda- 

 tion and is somewhat vaguely defined as re- 

 ferring to " the more active and obvious wear 

 and tear carried on by the sea, by streams, 

 rivers and glaciers" (p. 15). Corrosion is 

 " eating away of rock due to chemical solu- 

 tion, hence ... is frequently used to denote 

 chemical denudation" (p. 16). On page 34 

 the same writer refers to corrasion as " a 

 variant of corrosion used by some writers to 

 denote the vertical excavation performed by 

 a stream," but recommends the use of erosion 

 in its place. 



The above quotations are only a few out of 

 many and include only books published in the 

 last ten years, yet they show an amazing diver- 

 gence of views. This will be better illustrated 

 by the following summary: 



Out of a total of 7 authors quoted. 

 Denudation is defined by 5: 



as weathering plus removal, transporta- 

 tion and deposition of rock frag- 

 ments; (1) 

 as a general term for the lowering of tlie 



land surface by any agency; (2) 

 as the removal of loose mantle rock (1) 

 as weathering plus the mechanical work 

 of water (1). 

 Erosion is defined by 6: 



as removal, transportation and deposition 



of rock fragments (1) 

 as the formation and removal of rock 



debris (3) 

 as the mechanical work of water (1) 

 as rock wear plus solution (1) 

 as the more active and obvious wear and 

 tear performed by streams, rivers and 

 glaciers and by the sea (1) 

 Corrasion is defined by 6: 

 as mechanical degradation by rivers (2) 

 as the detachment of rock particles by 



wear of any agent (2) 

 as the wear caused by wind-driven par- 

 ticles (2) 

 One author rejects the term altogether, 

 holding it equivalent to erosion. 

 Corrosion is defined by 4: 



