SUMMAKY 415 



history of which sho^vs a yci-}' notable faihire of unanimity in nsage. A 

 review of the older definitions has led to one whicli is verbally new, Init 

 covering essential ideas underlying Lyell's use of '^metamorphie/' and is 

 nearly the same as ITarker's (1889, page 15) formal definition. 



The exj^ediency of that definition appears cleai-er after meanings have 

 been properly assigned to sncli expressions as "^regional/' "local/' "dy- 

 namic," "static," and "contact" metamorphism. Some of their respective 

 13ublished definitions can not l:>e f idly adopted without logical conflict with 

 the preferred definition of metamorpliism itself; yet the necessary de- 

 partures from authority are, in general, not any more serious than if one 

 tiles to use these older terms in any other systematic, logical subdivision 

 of metamorphic processes as now understood. 



The existing terminology does not suffice to cover all the categories. 

 Thus static metamorphism includes what are here called "stato-hydral 

 metamorphism"' or hydro-metamorphisui, and ^'stato-thermal metamor- 

 phism" or load metamorphism. Dynamic metamorphism is divided into 

 "dynamo-hydral metamorphism" and "dynamo-thermal metamoi'phism." 

 Metamorphism of rocks, produced by their burial under overthrust masses, 

 is called "dynamo-static metamorphism." Metamorpliism by a coinl)ina- 

 tion of igneous injection and deep burial is distinguished as "load-contact 

 metamorphism." 



The question whether pure volatilization is technically a metamorphic 

 process seems to be best answered in the negative. 



How the proposed scheme may meet the needs of working geologists is 

 a question briefly discussed. Those occupied with the Precambrian com- 

 plexes are apt to find the expressions "load metamorphism" and "load- 

 contact metamorphism" at least as useful as '^regional metamorphism" or 

 "dynamic metamorpliism." The classification is fairly elaborate, but it 

 will seldom be incumbent on the field investigator to consider the sub- 

 divisions of dynamic metamorphism or to apply the term "dynamo-static 

 metamorphism." The memory must, however, be somewhat iMirdened in 

 the use of any workable classification. 



The suggested scheme does not interfere with the ein];)loynient of cer- 

 tain descriptive words, which for various reasons have no place in it. 

 Those appearing in the table of classification have been systematized in 

 meaning, with a double object: first, to express the just conclusions of 

 the present day as to genetic conditions; secondly, to furnish a s^chcmc 

 elastic enough to admit further discovei'ies about the origin of the ci-ystal- 

 line schists, without seriously dislocating the partial classi Heat ion so far 

 erected. 



The relation of metaniorphisiu to "ultra-iiK'tamorphism" has been con- 

 sidered, j.oarl mctainorpliisin, load-contact metaniorphisra, and [tossibly 



