PETROGRAPHICAL CHARACTER OF AJIBIK QUARTZITE. 289 



the area. In an intermediate phase, while eonchoidal fractures are seen 

 they do not wholly separate different jjarts of the rock, so that what would 

 have been separate fragments had the fractures gone further are but a half 

 or a third separated from the (][uartzite background. The most extremely 

 alterated quartzite, instead of being brecciated, was mashed throughout, 

 and as a result passed into a biotitic or muscovitic quartz-schist, or into 

 coarse, completely crystalline, typical chlorite-schists, biotite-schists, and 

 muscovite-schists. 



In the northern and eastern parts of the district the quartzites grade 

 upward l)y interstratiti cations into the Siamo slate. In the southern and 

 southwestern parts of the district the formation grades in a similar man- 

 ner into the nonfragmental Negauuee iron formation. In sees. 27 and 28, 

 T. 47 N., R. 27 W., the intermediate phases are slates like those of the 

 Siamo formation. 



Microscopical. — Wlicre tlie Ajibik quartzite rests upon the Archean, and 

 therefore has a conglomerate or feldspathic quartzite at its base, it is very 

 similar to the basal conglomerates of the Mesnard quartzite and Wewe slate 

 described on pages 224-227, 263-265. The basal rock in some places is 

 a distinct conglomerate, and in others is composed mainly of the separate 

 mineral constituents of the adjacent underlying rocks. At many places 

 the basal horizon has been so much mashed as to pass into a crystalline 

 schist. In these places, instead of the conglomerate, we have chloritic, 

 sericitic, biotitic, or muscovitic schists, and in the most extreme stage of 

 alteration the rocks pass into typical mica-schists, the leaflets of biotite and 

 muscovite being of large size and having a parallel ai-rangement. In this 

 phase the quartz grains are wholly granulated; the new quartz which has 

 developed is similar in appearance to the granules; and the original feld- 

 spar is wholly decomposed, its place being- taken by the muscovite, biotite, 

 and secondary quartz. In certain of the schist-conglomerates, while the 

 matrix is completely crystalline, in hand .specimens the mashed and greatly 

 elongated conglomerate pebbles may still be recognized. 



Where the formation underlying the Ajibik quartzite is the Wewe slate, 

 there are apt to be interlaminated with the lower horizons of the quartzite, 



MON XXVIII 19 



