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KiNAHAN — Metamorphic Action. 471 



may be at certain times tinder the influence of metamorphic action ;. 

 while at other times the same area of rock-masses may be at or near 

 the surface of the earth, and so removed from metamorphic action ; 

 the limit of the number of distinct times the rock-masses underwent 

 metamorphism being the number of separate times when they were 

 buried under a sufficient thickness of accumulations to bring about 

 conditions favourable to the different invasions of metamorphic action. 

 This leads to the question, What effect or effects wotdd each of these 

 distinct aggressions of metamorphic action have on the rocks ? 



The different degrees of alteration that rocks exhibit in some 

 metamorphic areas appear to suggest that, in an area of regional 

 metamorphism, the action may be much more intense about certain 

 centres than elsewhere ; so that, in such areas, rocks will be fonnd 

 graduating continuously from unaltered through " sub-metamorphic 

 rocks," schist, and gneiss, into granitic gneiss and metamorphic 

 granite. It therefore appears allowable to suggest that each succes- 

 sive accession of metamorphic action into a larger or smaller portion of 

 a tract of rocks will intensify the preceding effects of metamorphism 

 respectively ; and if the intensity of the metamorphic action is about 

 the same at each return of it, the changes due to each will be some- 

 what similar to those in a district like that just mentioned; that is, 

 sub-metamorphic rocks will be changed into schist, schist into gneiss, 

 gneiss into granitic gneiss ; but granitic gneiss will become more 

 coarsely crystalline, the plates of aggregated crystals of each mineral 

 becoming larger and more conspicuously developed. If the action be not 

 intense enough, the granitic gneiss cannot be changed into metamorphic 

 granite ; while, on the other hand, metamorphic granite may change 

 into granitic gneiss by foliation being developed in it ; as, for instance, 

 the metamorphic granite of Gal way, if subjected to metamorphic 

 action, should change into a coarse gneiss, like the " Labradorians " of 

 Ha-Ha Bay, province of Quebec.- 



From the literature of the subject, it would appear that many 

 observers, otherwise good geologists, have a vague idea that metamor- 

 phic rocks must have been originally sedimentary or allied rocks, more 

 especially those that now have a marked schistose or gneissose folia- 

 tion. Such a supposition, however, must be more or less modified, as 

 on examination it will be found that, associated with all sedimentary 

 rocks, no matter what their age, there may be found as adjuncts 

 greater or less intrudes of granitic, felspathic, or basic eruptive rocks, 

 which necessarily will be metamorphosed along with the sedimentary 

 rocks. Also, mature consideration will demonstrate that even inde- 

 pendent tracts or areas of granite, felstone, or whinstone, if placed 



* Kocks, according to their chemical composition and physical properties, are 

 unequally affected by metamorphic action. This must be understood, as it is too 

 long a subject to be entered into here. Therefore, in the sub-metamorphic rocks 

 and schist, subordinate gneiss will probably be found ; while in the gneiss, granitic- 

 gneiss, and even the metamorphic-granite, there may be subordinate schists. 



