R. A. Daly — Secondary Origin of Certain Granites. 195 



The profound macroscopic differences of aspect are imperfectly 

 illustrated in fig. 3, which shows the variation of color-tint. 

 The corresponding variations in the specific gravity of speci- 

 mens taken in the cross-section of the sill is shown in the fol- 

 lowing table : 



Locality of specimen. Sp. gr. 



1 5 feet from upper contact 2*773 



40 " " " " __. 2-784 



50 " " " " 2-800 



Average for granite zone about 2*790 



200 feet from upper contact _ _ . 3-020 



Average for middle of sill about 3*025 



200 feet from lower contact 2-967 



30 feet " " " 2-980 



A series of determinations showed in addition that the average 

 specific gravity of the normal gabbro in all the sills of the 

 Boundary belt is about 3-020. 



Exomorphic contact action was observed at both upper and 

 lower contacts with the Kitchener quartzite. It has taken the 

 form of increasing the already high induration of the sedi- 

 ments with an accompanying special development of biotite at 

 both upper and lower contacts. Though there is evidence of the 

 feldspathization of the quartzite at the upper contact, none has 

 yet been forthcoming for the lower contact, where, neverthe- 

 less, feldspar may have been similarly introduced from the 

 magma. Doubtless on account of the chemical nature of the 

 invaded sediments contact metamorphism is not conspicuous 

 in the field, nor is it easy to trace its influence. The writer's 

 impression is that the effects are more manifest, the action 

 having been more intense, at the upper contact than at the 

 lower, but additional field study will be required to test the 

 real truth of that impression. 



Apart from the development of exotic feldspar in the quartz- 

 ite, indications of true pneumatolytic action seem to be lack- 

 ing at both contacts. Mineral veins, including quartz veins, 

 except occasional stringers of quartz, are conspicuously absent. 



Field Hypothesis. — The hypothesis adopted in the field 

 to explain these rocks and their relations involved a secondary 

 origin for the granite-grauophyre zone at the top of the sill. 

 That zone was thereby interpreted as due to the contact-action 

 of the gabbro intrusion on the adjacent Kitchener quartzite ; 

 digestion and assimilation of the sediments both on the main 

 or "molar" contacts and on the peripheries of blocks shattered 

 off from those contacts, was credited with the formation of a 

 new compound magma from which the highly acid and some- 

 what anomalous granite was derived. The fact that the acid 



