408 R. A. DALY— METAMORPIIISM AND ITS THASES 



by vapors and true gases, from the domain of pneumatolysis causes a 

 highly arbitrary and apparently quite unnecessary departure from the 

 literal meaning of ^^imeumatolysis"; lastly, because connate fluids of the 

 country rock, which have metamorphic effects very similar to those ex- 

 erted by magmatic emanations, are not considered in the subdivision. 



According to another conceivable classification, ''TiydrotJiermal-coniact 

 metamorphism" might be defined as metamorphism controlled by water 

 and its accompanying vapor, while '' pneumatolytic-contact metamor- 

 phism'^ includes the types of contact metamorphism controlled by other 

 volatile substances. ''Thermal-contact metamorphism" would be defined 

 as before. This scheme also is hard to apply in nature, and it does not 

 agree with the concept of pneumatolysis which, vague as it is, now rules 

 in the minds of most geologists. 



In the present connection it should again be noted that existing criteria 

 do not in many cases suffice to distinguish,- practically, "thermal-contact" 

 effects from those controlled by gases and vapors. 



In fact, the writer has been unable to find in the literature any sugges- 

 tion of a satisfactory subdivision of the contact-metamorphic processes. 

 The importance of the subject in the theory and description of ore de- 

 posits is manifest. Perhaps the group of economic geologists will yet 

 develop a truly scientific classification, mth corresponding definitions, 

 for the phases of contact metamorphism. 



DEFINITION AND SANCTION OF ''LOAD -CONTACT METAMORPHISM" 



Extensive masses of the older Precambrian rocks are composites of 

 sediments or surface volcanics with injections of igneous material. Gen- 

 erally granitic in composition, the intrusives commonly favor the form 

 of the sill or laccolithic sheet. So numerous are these bodies that their 

 total contact-metamorphic effects are profound. In some cases tiie 

 changes wrought are those of pure contact-metamorphism. In very many 

 others the recrystallization of the invaded formation has been simulta- 

 neously controlled by the weight of its cover. The influences of vertical 

 stress, of the earth's general heat, and of the injected magma are thus 

 concurrent. New crystallizations in the country rock are caused hy a 

 combination of causes vjhich may be called ''load,- contact metamorphism," 



The writer first began to appreciate this compound type while studying 

 the Shuswap terrane of British Columbia, with which a comparison of 

 Precambrian rocks in Ontario was later made in the field. Where the 

 siliceous sediments, limestones, and basic volcanics of the western ter- 

 rane are not charged with igneous injections in great number, the rocks 

 are seen to have been recrystallized by pure load-metamorphism. In 



