DEFINITION OF METAMOEPIIISM 377 



HISTORY OF THE WORD "METAMORnilSiM" 



In the first edition of his "Principles of Geology" (1833, volume 3, 

 page 374) Lyell introduced ''^metamorphic"^ to geology. Neitlier there 

 nor in later editions does he appear to have used the noun "metamor- 

 phism/^ but he wrote of "metamorphic rocks" and the "metamorphic 

 theory/' The very invention of the Greek compound, identical in litei-al 

 meaning with the familiar "transformed" or "transforming," shows from 

 the start that Lyell did not intend his word to cover all rock alterations. 



He later elaborated the theor}'- of metamorphism, in his "Elements of 

 Geology" (1838), stating (page 219) : "Metamorphic . . . expresses 

 a theoretical opinion that . . . strata, after having been deposited 

 from water, acquired by the influence of heat and other causes a highl}- 

 crystalline texture." On page 23 one reads: "It is true that all meta- 

 morphic strata must have been deposited originally at the surface, or on 

 that part of the exterior of the globe which is covered by water ; but, ac- 

 cording to the views above set forth, they could never have acquired their 

 crystalline texture unless they had been modified by plutonic agency undei' 

 pressure in the depths of the earth." He continued (page 379) : ^'The 

 metamorphic rocks must be the oldest — that is to say, they must Jie at 

 the bottom of each series of superimposed strata — ^because the iniliience 

 of the volcanic heat proceeds from below upwards." 



From the sixth to the last edition, Lyell's "Principles" contained tlie 

 following passage, giving a virtual definition of metamorphism : "The 

 transmutation [of fossiliferous strata into such rocks as gneiss, mica 

 schist, or marble] has been efi:ected by the influence of subterranean heat 

 acting under great pressure, and aided by thermal water or steam and 

 other gases permeating the porous rocks, and giving rise to various chem- 

 ical decompositions and new combinations" (sixth edition, 1810, London, 

 volume 1, page 320). 



Lyell early adopted the view that magmatic and connate gases and 

 vapors, as well as mere heat, are important agencies in metanior])hisni 

 ("Elements of Geology," 1838, page 246). 



Durochcr (184G, page 546) defined metamorphism as (translated) 

 "the sum of the effects of transformation, of change of natui'e or textui'e, 

 wliich the rocks composing the earth's crust have undergone." He 

 pointed out that metamorphic rocks are most developed in regions affected 

 by crustal deformation, though also habitually along igneous contacts. 

 He laid great sti-ess on the influence of heat in metamorpliism, yet classi- 

 fied as metamorphic se\ei'al types of change at ordinary temperatures, 

 including oxidation and liyd ration of rocks by weathering, as well as 

 concretionary action. 



