386 R. A. DALY iMETAMOKPlllSM AXl) 1T8 PJIASES 



l)r()a(lcr di'dnitioii of iiietainorj)lii>siii to niatcli the needs of general geology. 

 I'irsson jecognizes two kinds of metaniorpliism, contact and regional, the 

 latter covering (page 319) dynamic metamorphism '^as a prononnced 

 phase of it in one direction." He nses the expression "constructive meta- 

 morphism" for Van Hise's '^anamorphism'' — that is, recrystallization in 

 the '^zone of rock-flowage" — but adds (page 316) that simple downward 

 ])ressnre, "static pressure . . . appears to have little altering effect 

 on rocks." 



Eies and Watson (1915, pages 200-201) follow A"an llise in defining 

 metamorphism "as any change in any rock, regardless of origin," thus 

 including ^veathering changes in "alteration or metamorphism proper." 

 They hold that static metamorphism and pressure metamorphism both 

 "refer to quiescent conditions." 



In their new book, Leith and Mead write (1915, page xvii) : "Eock 

 metamorphism is here defined to cover all mineralogic, chemical, and 

 physical changes in rocks subsequent to their primary crystallization from 

 magma. . . . We shall follow Van Hise by including under metamor- 

 phism not only development of schistose and crystalline rocks, but also 

 all changes involved in rock weathering and cementation." 



F. W. Clarke (191G, page 583) introduces his chemical discussion of 

 the subject thus: ^'In its widest sense the adjective metamorphic may be 

 applied to any rock that has undergone any sort of change. Practically, 

 however, it is used to describe a well defined class of rocks in which the 

 transformation from an original form has been nearly complete. A 

 slightly altered igneous or sedimentary rock is not commonly called meta- 

 morphic ; neither is a mass of decomposition products so designated. . . . 

 Some varieties of metamorphism are entirely physical or structural, and 

 therefore will not be considered in this memoir. Metamorj^hoses which 

 represent only a development of slaty or schistose sti'ucture are of. this 

 kind. In most cases, hoAvever, metamorphism is accompanied l)y chem- 

 ical changes, which are indicated by the production of new minerals, and 

 this sort of metamorphism concerns us now. It may be regional, when 

 large areas are affected, or a phenomenon limited to a contact between 

 two reacting rocks; but these distinctions are of little significance cliem- 

 ically." 



Dictionaries and encyclopedias reflect a diversity of usage similar to 

 that illustrated in the foregoing extracts from standard works ou geology. 



The 1895 edition of the Century Dictionary (Xew York) gives this 

 definition of metamorphism : "The process of metamorphosing or chang- 

 ing the form or structure; specifically, chemical change and rearrange- 

 ment of the coDstitnents of a rock by wliich they are made to assume new 



