Reviews — Professor Daly on Metamorphism. 223 



"waters that are permanently radioactive, since they contain radium 

 salts in solution. In addition to this, however, there is a temporary 

 radioactivity, due to tlie presence in solution of tlie. short-lived gas 

 radium-emanation picked up from the formations through which the 

 water has passed. In this respect the Alberta springs are by far the 

 most valuable, though the actual figures still fall below those for 

 Bath and Buxton. The report is illustrated with a map showing the 

 situation of the springs investigated and with twenty-one photographs 

 of the springs themselves. 



AiiTHUR Holmes. 



IV. — Metamokphism and its Phases. By li. A. Daly. Bull. 

 Geol. ISoc. Am., vol. xxviii, pp. 375-418, 1917. 



PKOFESSOR DALY has written a paper oa the use and meaning 

 of the term metamorphism, and on the classification of the 

 various processes which give rise to metamorphic rocks, for which all 

 students of geology may well be grateful. The definition advocated 

 by the author is as follows : Metamorphism is the sum of the processes 

 ivhich, working leloiv the shell of tveathering, cause the recrystallization 

 of the original crystalline materials in rocks (with or without chemical 

 reactions) or the crystallization of original amorphous materials in rocks, 

 the change in each case not being accompanied hy a general melting of the 

 rock or hy general simultaneous solution of its constituents. 



All weathering processes are thus cut out, for their inclusion (an 

 attempt at which has been made by Van Hise and more recently by 

 Leith and Mead) leads to a gigantic subject of unmanageable 

 proportions, and one for which the term metamorj^hism ceases to have 

 its restricted, and therefore most useful, traditional significance. 

 Volatilization is also excluded, examples of this type of transformation 

 being the change from mud to shale, or lignite to coal, and coal to 

 anthracite. Daly's definition, however, does not clearly distinguish 

 between alteration processes of exogenetic origin and those of 

 endogenetic origin, for some cases of cementation, of recrystallization 

 of limestones by phreatic waters, and of metasomatism by descending 

 solutions, are clearly included as examples of metamorphism accoiding 

 to Daly, although they may be indubitably the result of exogenetic 

 processes. It seems to the present writer, following the lead of Mr. T. 

 Crook, whose paper on the genetic classification of rocks (Min. Mag., 

 vol. xvii, p. 55, 1914) is one of the most illuminating recent 

 contributions to petrology, that the term metamorpliism should be 

 still further restricted so as to exclude all alterations due to 

 exogenetic processes. 



The classification of metamorphic processes suggested by Daly is 

 as follows : — 



A. Regional Metamorphisji (not caused by eruptive bodies). 



I. Statjc Metamokphism (organic movement not a causal 

 condition). 



1 . (Temperature low) Stato-hydral or hydro metamorphism. 



2. (Temperature high) /S^a^o- ^//^/wm/ or /off(/ metamorphism. 



