130 



SCIENCE 



[N. S. Vol. LIV. No. 1389 



DISCUSSION AND CORRESPONDENCE 



" DENUDATION," " EROSION," " CORROSION " 

 AND " CORRASION " 



The recent article by Professor M. H. Bis- 

 sell ^ on the use of the terms " denudation," 

 " erosion," " corrosion " and " corrasion " ex- 

 presses a need which is felt by most instructors 

 of elementary classes in geology and physi- 

 ography. In the opinion of the writer the 

 confusion of terms is attendant on a confusion 

 of ideas concerning three very essential topics 

 discussed in any elementary class, namely, 

 weathering, denudation, and deposition. 



The geologic agents of denudation and depo- 

 sition are practically identical. Hence it is 

 logical to discuss the denudational and depo- 

 sitional work of the wind, running water, un- 

 derground water, the ocean, ice, and gravity. 

 It seems to the writer, however, that the prac- 

 tise of placing a discussion of weathering in a 

 chapter entitled " The work of the atmo- 

 sphere " is very confusing. The agents of 

 weathering are quite distinct from those of 

 denudation and deposition, and require sepa- 

 rate treatment. It is very difficult to show the 

 connection between the work of the atmo- 

 sphere and exfoliation. It is poor physics to 

 teach that the expansion and contraction of 

 rocks is due to the atmosphere. 



The writer would define weathering as the 

 alteration of rocks rendering them liable to 

 transportation by the dynamic forces having 

 their origin near the surface of the earth. 

 Wind, water, ice, and gravity can not trans- 

 port bed-rock. But when bed-rock is broken 

 down by the chemical and mechanical activity 

 of weathering, its particles may be transported. 



In a similar way, denudation might be de- 

 fined as the removal of the products of rock 

 weathering by the dynamic forces having their 

 origin near the earth's surface. The process 

 involves the lowering of the earth's surface by 

 the combined actions of erosion and trans- 

 portation. Erosion may be subdivided into 

 two processes: (1) the mechanical wearing 

 away of rocks (abrasion) by wind, running 

 water, ice, and gravity; and (2) the chemical 

 loss (corrosion) due to agents present in pass- 

 ing streams of water and air. The central 



1 Science, April 29, 1921. 



idea expressed by the term " denudation " 

 should involve the erosion and transportation 

 of rock debris from its source to a position be- 

 low baselevel. 



The word " corrasion " appears to be so 

 similar in usage to the term " erosion " that 

 it should be discarded in favor of the com- 

 moner term. 



The writer believes that the average geolo- 

 gist has not departed very far from the root 

 significance of the terms discussed by Profes- 

 sor Bissell. The development of the term 

 " weathering," however, has outrun its origi- 

 nal meaning, and processes are included which 

 are not connected with atmospheric action. 



A diagrammatic outline for class discussion 

 of these topics might be the following. 



("Mechanical (frost) 

 ■^y^^^gj J Chemical (hydration, 



[ oxidation, etc.) 

 Heat and cold, mechanical (ex- 

 foliation) 

 Atmospheric gases, chemical 

 (oxidation, carbonization, 

 etc.) 



(Mechanical (root 

 growth) 

 Chemical (acids from 



Weathering. . 





Animals. 



roots and decay) 

 Mechanical (dig- 

 ging, burrow- 

 ing; 

 Chemical (acids 

 from decay and 

 excreta) 



Denudation 



and 

 Deposition. . 



Wind: Erosion, transportation, 

 deposition 



Running Water : Erosion, 

 transportation, deposition 



Underground Water: Erosion, 

 transportation, deposition 



Ice : Erosion, transportation, 

 deposition 



Gravity: Erosion, transporta- 

 tion, deposition 



It may appear that the chemical activity of 

 water in weathering, and of running water in 

 denudation, are one and the same thing, but it 



