CLASSIFICATION OF METAMORPfliC PROCESSES 393 



knows the fliffioiilt}' of framing (leflnitions whicli arc at once stahle in the 

 face of new discoveries and yet are of meaning intensive enough to matcli 

 contemporary knowledge. Subdivisions shonhl, moreover, be elastic 

 enough to take in metanior])hic phenomena whose causes are not now 

 fully understood. 



The problem of fruitful sulKlivision is specially insistent for field geol- 

 ogists. Hence the chissification to be proposed is primarily geological. 

 The dichotomous division into katamorphism and anamorphism may 

 prove to have very great value in the description of rock or ledge. But, 

 especiall)^ in field geology, an indication as to the controlling cause of a 

 solution al change generally means much more than does a mei'c indica- 

 tion as to how the reactions ran in the solution. If increase of pressure 

 is required to shift ecpiilibrium in a given direction, tlie origin of the 

 pressure increase may be immaterial to the student of the reaction as a 

 purely physico-chemical change. On his part the geologist is deeply con- 

 cerned with the cause of the pressure increase. Is it due to orogenic 

 compression or to burial under a thickening cover of sediments or vol- 

 canic rocks? The professioiial injunction to answer such a fundamental 

 ([uestion should be reflected in the main classification. If heightened 

 temperature is the chief cause of new crystallizations in a rock, the geol- 

 ogist must go further than the plnsical chemist and ask whether the 

 lieating lias resulted from the proximity of igneous masses or from oro- 

 genic crushing. In general, the physical chemist may be content with 

 tlie laboratory report that a certain rock has been developed by anamor- 

 phic processes; the geologist is much more interested in the condition 

 of the earth's crust whicli has led to that anamorphic assemblage of 

 minerals. 



The prevailing classifications do, in fact, aim to meet this chief re- 

 quirement on the part of the geologists. The deeper meaning of meta- 

 morpiiism as one aspect of tlie development of the globe as a whole has 

 given life to such ^videly used terms as "dynamic metamorphism," "static 

 metamorphism,'' "load metamorpliism,'- and "contact metamorphism.'' 

 So firmly fixed are these, no acceptable classification is henceforth likely 

 to dispense with most of them, if indeed with any of them. 



Unfortunately, each of the names already given to different phases of 

 metamorphism has, like tlie key word itself, had varying definitions. It 

 becomes necessary in each case to decide what is the most advisable defi- 

 nition ))efore incor})()rMting tliat term in the system. 



As a general rule, the phases of metamorphism have been given names 

 that must be arbitrarily defined, else they would have been nearly or quite 

 useless. Oljjcction lias been made to "dynamic metamorphism'^ on the 



