DEFINITION OF METAMOKL'HISM obU 



self, favored the exchision of weathering from the group of mctamorphic 

 processes. Among the few who do not follow tradition is Van Hise, wlio, 

 however (1904, page 163), emphasizes ^^the fact that the alterations in 

 the helt of weathering are very different from [those in] the belts heloAv/' 

 He continues : "In man}-^ places the change in the character of the altera- 

 .tions in i3assing from the helt of weathering to the belt of cementation is 

 very sudden/^ Leith and Mead likewise distinguish a "belt of weather- 

 ing" (perhaps better called a. shell of Aveathering) wliercin rock altera- 

 tions are more or less sharply distinct from those induced at greater 

 depths in the earth. The suggestion of Lindgren and otliers that the 

 alterations properly referable to Aveathering are confined to the earth sliell 

 above the Avater table ia a more precise expression of the same general 

 idea and is Avorthy of special consideration as a possible criterion for dis- 

 tinguishing metamorphic changes 'from Aveather changes. ^J'hai, there are 

 transitions between the sets of conditions leading respectively to (ordinary 

 rock-weathering and to the development of cei'tain crystalline scbists is 

 not a compelling consideration. The existence of transitions in ]uost 

 natural phenomena onglit not to, and does not, discourage the effort to 

 classify. In their fundamental division of rock changes into katamorphic 

 and anamorphic, A^an Hise and liis followers have not been deterred by 

 the fact that both of these classes of alterations are displayed within the 

 limits of a single rock body or even AAdthin the limits ot an origiii<'il roek- 

 forming mineral. The loA^-er boundary of the shell of weathei'ing is cer- 

 tainly at least as definite as the boundary betAveen the katamorphic and 

 anamorphic shells. 



The chief reason for the exclusion of weathering processes is, oi' course, 

 to save the Avord "nietamorphism,'' to prevent its overburdening. As nsed 

 by the Van liise school, it is equivalent to "alteration, '^ and the more 

 recondite word becomes practically useless. If these anthorities Avere fol- 

 loAved in this matter, tAvo most useful Avords Avould become as niniecessary 

 as they are, respectively, in Van Hise's "Treatise on Metamorphism'^ and 

 Leith and Mead^s "]\retani()i;phic Geology." Oji the other hand, there is 

 the utmost need for "metaniorphism" as a designation for rock changes * 

 in depth, having nothing directly to do with Aveather alteration. 



AVeatheriiig processes already demand whole volumes for their sum- 

 marizing. The geological ])rofession is not likely to agree Avith the ])ro- 

 posal to consolidate that immense subject Avith the yet vaster one relating 

 to rock changes under conditions of high pressure, high temperature, or 

 b'tth : nor ;nv most gc()h)_oisis to he attracted by a definition of "meta- 

 morphic iocIns/' ■^vhich are theicby nuule to include residual clays and 

 soils, glacial deposits, shale, limestones, etcetera.' Leith and Mead (1915, 



