July 1, 1921] 



SCIENCE 



13 



DISCUSSION AND CORRESPONDENCE 



USE OF THE TERMS " EROSION," " DENUDA- 

 TION," " CORRASION " AND " CORROSION " 



I AM interested in Mr. Bissell's plea for a 

 more precise term, in geological literature, of 

 the terms, " erosion," " denudation," " corro- 

 sion " and " corrasion." Without entering in- 

 to a discussion of the merits of various past 

 definitions of these words, may I presume to 

 express my own views on this subject? 



" Erosion " means " gnawing away," and is 

 properly used to include all natural processes 

 which have their origin at the earth's surface 

 and which involve the destruction of rocks at 

 or near the earth's surface. This is the broadest 

 term referring to surficial rock destruction. It 

 embraces work performed by passive or motion- 

 less agents (weathering) and work performed 

 by moving agents, such as running water, 

 glacial ice, waves, and wind. It may be used 

 correctly for rock destruction on the land or 

 on the sea floor. Thus, we may speak of ero- 

 sion of the sea floor by waves or by submarine 

 currents, and of the erosion of rocks, exposed 

 on land, by moving ice or by alternate contrac- 

 tion and expansion due to heating and cooling, 

 etc., etc. While it must connote transporta- 

 tion and may connote deposition, it should not 

 be used to include these dependent processes. 



" Denudation," by derivation, refers specifi- 

 cally to stripping or laying hare. It is often 

 used in the sense of natural removal of soil or 

 mantle rock from underlying solid rock, or re- 

 moval of one rock formation from one lying 

 below. It refers to erosional processes which 

 are destructional, and like erosion should not 

 be used to denote transportation or deposition. 

 Almost, if not quite, without exception, " de- 

 nudation " refers to stripping (erosion) only 

 on land, whether it is on a small scale or on 

 a large scale. 



" Corrasion " is mechanical erosion per- 

 formed by moving agents such as wear by gla- 

 cial ice, by wind, by running water, etc. 



" Corrosion " is most commonly used for 

 chemical erosion, whether accomplished by 

 motionless or moving agents. 



I have suggested the foregoing definitions 

 always having in mind that the " rock " eroded 



may be consolidated or unconsolidated and 

 that corrasion is accomplished largely by vir- 

 tue of sand, silt, or other rock debris carried 

 by the moving agent of erosion. 



Frederic H. Lahee 

 Dallas, Texas, 

 May 11, 1921 



THE BREEDING HABITS OF AMBYSTOMA 

 TIGRINUM 



The eggs of Anibystoma tigrinum are 

 usually described as occurring in small 

 clumps. This is typical of the species in the 

 eastern part of its range. While collecting in 

 Colorado at an altitude between 6,000 and 

 7,000 feet, I found eggs of tigrinum laid 

 singly. When first laid the egg resembles 

 that of Diemictylus. As development con- 

 tinues the outer envelope becomes swollen 

 until at the time of hatching its diameter is 

 one half to three quarters of an inch. The 

 eggs are attached to vegetation or debris. 

 The depth varies from a few inches to two 

 feet. On one occasion adults brought into 

 the laboratory laid freely. 



Ralph J. Gilmore 



Colokado College, 

 Colorado Springs, Colo. 



a phenomenal shoot 

 An extraordinary water-shoot, discovered by 

 Mrs. B. W. Wells, near the city of Ealeigh, 

 N. C, on March 21, 1920, is of such unusual 

 size as to deserve recording. The shoot 

 sprang from the side of the trunk of a be- 

 headed tree of Paulawnia tomentosa (Thunb.) 

 Steud. and grew in one season (1919) to the 

 length of 19 feet, 5 inches. Twenty inter- 

 nodes were formed, the longest of which, 

 located a little below the middle of the shoot, 

 measures 19 inches in length. The base of 

 the shoot is 7.75 inches in circumference and 

 2.5 inches in diameter. Braunton in Bailey's 

 Encyclopedia of Horticulture gives 14 feet as 

 a maximum length of Paulownia shoots grow- 

 ing from the root after winter killing. The 

 shoot recently discovered, exceeding this by 

 5 feet, 5 inches, is believed to be a record for 



