?)92 R. A. DALY METAMORnilSM AND ITS i'llASES 



mud to partly (-Tystallijio limestone, devitrification^ and the consolidation 

 of sediments when accompanied l)y the formation of crystalline cement. 

 So defined^ in terms of the '"tid result of various processes^ "metamor- 

 phism" has still an enormous extension in geology. The classification of 

 those processes, founded on tlieir fundamental causes, is the next step in 

 a scientific description of metamorphism itself. Especially after one has 

 tried to form a rigorous, yet practical classification, he realizes the value 

 of restricting "metamor])]iism'' in the measure just suggested. Only 

 witli completed analysis of this kind can the full meaning and validity 

 of the necessarily abstract definition be appreciated. 



Definition" of "metamorphic Rock" 



Perhaps a word as to its relation to the expression "metamorphic rock" 

 may not be out of place. Every modern writer holds that not all rocks 

 in wliich metamorpliic processes liave operated are to be technically called 

 "metamorphic." Irrespectively of tlie definition of the key word, the 

 class of metamorphic rocks groups only those that result from essential 

 change in tlie l)ody of each original rock. According to the proposed 

 flefinition of metanior])]iism itself, this cliange must involve new crystal- 

 lizations wliicli are distributed tlirough the rock body as a whole, affect- 

 ing most or all of it. Tlie criterion for a metamorphic rock is therefore 

 rloulde. When applied to tlie classification of rocks, there appears to be 

 confiict with common usage in only one respect. A volcanic glass, com- 

 pletely de vitrified in depth because of influences other than those con- 

 nected with its original magmatic state, is logically to be assigned to the 

 metamorphic rocks. This departure from tradition is intrinsically not a 

 ^^ery serious matter, on accouut of the small volume of glass in the earth^s 

 crust. The assigmuout may l)e defended on the ground that a bed of 

 graphite, derived from another kiud of amorjihous material, is regularly 

 ])ut ill the class of metamorphic rocks. 



Classification" of metamokpiiic Processes 



requirements of a working classification 



An ultimate systematizatiou of metamorphic pliases would be based 

 strictly on the origin of these alterations. Since the conditions of change 

 are not fully known, the ideal is yet to l)e reached. The best classifica- 

 tion now possible could represent no more than a summary report of 

 progress in interpretation. Nevertheless, it should embody the minimum 

 number of terms and definitions which can not apply after an indefinite 

 expansion of knowledge on the subject. Every student of metamorphism 



