380 R. A. DALY MKTAMOKPHISM A>sl) ITS PHASES 



for example, the cementation of loose sand by the infiltration of calcinm 

 carbonate furnishes a metamorphic rock, and a somewhat hydrated lava is 

 metamorphic. However, according to general usage, only those rocks are 

 called metamorphic in which alteration has reached a higher degree and 

 has taken the form of new crystallizations. He gave pseudomorphism as 

 a type of metamorphism with this narrower definition. He treated 

 diagenesis as a ^^Mittelweg" between metamorphism and original "Bil- 

 dung." Weathering was excluded from the ]:)liases of metamorphism. 



Phillips (1885, page 356 if ) described as metamorphic "all those parts 

 of aqueous strata which have been transformed in structure or appearance 

 by subterranean heat, or heat developed by pressure applied since their 

 deposition.'' He recognized two kinds of metamorphism : diiiciiiral , due 

 to l)urial (pressure and heat) or to orogenic movement (as in the origina- 

 tion of cleavage), and molecular, not expressly defined, but illustrated 1)y 

 the conversion of "earthy" carbonate of lime into marble. 



According to Prestwdch (1886, page 397), "metamorphism is that mol- 

 ecular and structural change in the strata of the sedimentary series, or in 

 the rocks of igneous origin, whereby they have undergone a transforma- 

 tion in the chemical combination of their elements, in mineral constitu- 

 ents, and in structure, so that their original condition has l)een more or 

 less modified and altered and their characters disguised." He points out 

 that this is in a sense true of all stratified rocks, since nearly all have 

 been changed ])y cementation, segregation, infiltration, or pressure. He 

 concluded that the term "metamorphism" should be restricted to '^those 

 greater chemical and mineral changes, caused by heat combined with 

 pressure and moisture." Prestwich distinguished contact, regional, and 

 normal metamorphism. 



Eegional metamorphism includes (1885, page 425, and 1886, page 408) 

 "changes effected by the agency of the physical causes to which Mr. ]\IaUet 

 referred the fusion of the volcanic rocks, namely, ilie heat produced locally 

 irUJiin ilie crust of the cartli hy transformation into lieat of tJie nircJian- 

 ical worl: of compression, or of crusliincj of 'portions of ttiat crust." In 

 tlie 1885 paper, page 425, he wrote: "N'ormal metamorphism I would 

 confine to signify, as hitherto, the changes caused by the heat due to 

 depth, on the supposition of the existence of a heated central nucleus of 

 the earth." However (page 430), "normal metamorphism depends not 

 so much on high temperature as on pressure and the presence of water." 

 To its operation he attributed (1886, page 413) the larger and more com- 

 mon class of metamorphosed strata, the alteration of which has been due 

 to a cause more general than igneous-rock heat (contact metamorphism) 

 or orogenic-crush heat (regional metamorphism). This general cause is 

 the internal heat of the earth, which becomes efficient only after burial. 



