STRUCTURAL GEOLOGY. 31 



Facts of petrographic significance.] 



between those gneisses which are plainly the result of a change in bedded frag- 

 mentals, and others which are nearly massive and homogeneous, but only present a 

 gneissoid structure, i. c., a prevailing direction of elongation of the constituent min- 

 erals. But it was found that such a distinction, while evident enough at the extremes, 

 could not be maintained, and that the bedded structure gradually loses itself, under 

 varying conditions, and passes into a simply gneissoid structure, the two being 

 parallel and plainly different phases of the same fundamental transformation. In 

 nearly all cases, where such a structural transition takes place it is observable also 

 that there is a corresponding transition in the mineralogical composition, the mass 

 of the rock becoming also more siliceous and alkaline. Indeed, there are consider- 

 able areas in the northern part of the state, occupied by crystalline rocks, which 

 would be assigned in mapping by one geologist to the mica schists, and by another 

 to the gneiss and granite, the last two rocks being grouped together according to our 

 uniform practice. There is always evidence of more or less plasticity and of bodily 

 movement of masses of the rock, and of streaming out of all the original structures, 

 in the vicinity of transitions from the schist to igneous gneiss. 



That the gneiss here referred to, i. e., that which is massive, with only a miner- 

 alogical gneissoid structure or but rare and confused schistosity, is traceable into 

 true granite, is a well-known fact of field observation, which has been repeated by 

 numerous geologists. There is on this point very little, if any, difference of opinion. 



When the crystallized fragmentals are dark colored, with larger amounts of 

 mica and hornblende, the resultant gneiss or granite is also darker, but seems to 

 differ in no way as to genesis and structure, from the lighter-colored crystalline rocks. 

 They differ only in holding larger amounts of the dark minerals, and in their pro- 

 portionate amount of quartz. In such instances the massive rock is rather dioryte 

 than granite, and may be syenyte; and in some instances all stages of transition from 

 one rock to the other can be collected within a comparatively small area. 



(2). Facts of petrographic xit/nijicdncc. As a group these rocks, from mica 

 schist to granite, are characterized by much silica and varying amounts of alkaline 

 feldspars. They rarely acquire hornblende or pyroxene and biotite in sufficient 

 amount to warrant the adjective basic* When these minerals do exist in consider- 



* The recent investigations of Dr. F. D. Adams on the chemical composition of certain gneisses of the province of Quebec,t 

 have a bearing on this point. He has compared the chemical composition of several gneisses with that of a gneiss from Trembling 

 mountain, and a granite from Carlingford, Ireland, these standards being assumed as normal igneous rocks. He finds that the 

 Kiiris.-.cs Investigated, while having a higher content of alumina than the assumed standards, a lower per cent of alkalies and a 



a sediment of the nature of a tuffaceous deposit." Dr. Adams concludes that the other gneisses of this series may be considered 

 highly altered conditions of ancient sediments. 



These are important results. It might be asked, however, what has determined the standard composition of an igneous 

 rock, and whether the distinguishing chemical characters accepted by Adams will not be found to vanish when the comparison is 

 extended so as to include undoubted igneous rocks from other localities. This seems quite possible from the fact that one of the 

 selected gneisses of Dr. Adams' series is pyroxenic, and manifests such intermediate characters that it is to be considered as 

 possibly " an altered igneous rock." The gneisses selected are interstratifled in limestones and quartzytes so metamorphosed as 



iLaurentian area to the north and west of St. Jerome. Geological Survey of Canada, vii, 1894 [1896], Report J, pp. 93-112. 



