DEFINITION OF METAMORPHISM 387 



forms and made to enter into new combinations, the most important result 

 of these changes being that the rock becomes harder and more crystalline 

 in structure/^ 



According to the Standard Dictionary (New York, 1895), metamor- 

 phisni comprises "the changes that go on in rocks, due to recrystallization 

 of their constituents, either with or Avithout alteration in the chemical 

 composition of the mass. 'I'he most important agents of metamorphism 

 are lieatj m,oisture, or other mineralizing factor, and pressure." These 

 are made active either by intrusion of igneous masses or by dislocations 

 or movements of the earth's crust. Cementation by the enlargement of 

 mineral grains is regarded as a metamorphic process. Static metamor- 

 phism is described as including "changes produced largely by pressure 

 without great shearing or dislocation of the rock-masses." 



The I^ew International Dictionary (Xew York, 190-f) regards meta- 

 morphism as covering only the "profound changes" in rocks and excludes 

 weathering and decomposition. It recognizes only two kinds — contact 

 and regional. 



The N'ew English Dictionary (Oxford, 1908) defines metamorphism as 

 "the process of change of form or structure produced in a rock by various 

 natural agencies" and adds: ''Mclaiiiorpliic. Of a rock or rock forma- 

 tion ; that has undergone transformation by means of heat, j^ressure, or 

 natural agencies" — an extraordinary solecism ! 



Flett, in the 1911 edition of the Encyclopedia Britannica, defi]ies 

 metamorphism as '^the alteration of rocks in their structural or mineral 

 characters by AAhich they are transformed into new types." He excludes 

 weathering, decomposition, and cementation. He admits two kinds: 

 (1) "contact or thermal," and (2) "folding or regional," making no 

 mention of load or static metamorphism. 



DISCUSSION OF THE OLDER DEFINITIONS OF METAMORPHISM 



A review of the literature thus shows that "metamorphism" has been 

 used in at least five different senses. 



1. Authors defining it as including ''all changes" in rocks, after the 

 original embodiment of those rocks as distinct masses of material, are : 

 Durocher (1846), Kalkowsky (1886), Eeyer (1888), Van Hise (1904), 

 De Launay (1905), Kemp (1908), and Leith and Mead (1916). 



2. Authors excluding weathering processes, but including cementation, 

 are: Alrlet (1847), Lossen (1872), von Hauer (1878), Green (1882), 

 riiillips (INS.-,). Tcall (1888), Lawson (1888), Harker (1889), Eoth 

 (]Si)0), tlic writers in the Cent my Dictionary (1895) and Standard Dic- 

 tionary (1895), Dana (1895), A. Geikie (1903), the writer in the New 



XXVIII — BijLT.. Qkou Soc. A.m., Vor,, 28, lOlG 



