THE MESNARD QUARTZITE. 223 



PETROCiRAPHICAL CHARACTEK. 



Macroscopicai. — Petro^i'apliically the formation consists of conji'loinerates, 

 gravwackes and graywacke-slates, and qnartzites, witli all gradations 

 between the different phases, althongh quartzite is the predominant rock. 

 Where rocks of the formation are found in contact with or close to the 

 surrounding granite, they are a coarse granite-congloinerate, or a rock 

 wdiich may be called a recomposed granite where the constituent particles 

 composing the rock are the separate mineral particles of the Basement 

 Complex. 



On tlie south side of the Algonkian trough the cong-lomerate is mag- 

 nificently exposed west of Lake Mar}- in the 8W. ^ -"^ec. 9 and the SE. ^ sec. 

 8, T. 47 N., R. 25 W. It may also be well seen at and east and west of 

 the line between sees. 1 and 2 of the same townslh}), and at other places. 

 With the fragments of granite are apparently many of vein quartz, and a 

 few of red jasper, of chert, and of quartz-rock. In some cases this bowlder- 

 bearing granite-conglomerate passes into a less conglomeratic, reddish rock 

 which closely resembles the original granite. 



On the north side of the trough north of Mud Lake, in sec. 29, T. 48 N., 

 R. 25 W., the lowest horizon is again a basal conglomerate, the numer- 

 ous fragments being derived mainly from the granites and schists of the 

 Northern Complex, the latter being more abundant because immediately 

 adjacent. The fragments vary from those of minute size to bowlders 2 or 

 3 feet in diameter. Here no fragments of chert or jasper were found. 



The basal conglomerate at Mud Lake usually passes quickly into' inter- 

 stratified slate and graywacke, and then into a quartzite. The slate and 

 graywacke are very closely folded, there being many reduplications of 

 the same strata, all having, however, a southern monoclinal dip, and the 

 axes of the little folds pitching steeply. So close has been the compres- 

 sion that the moi-e resistant belts of graywacke in the slate have been 

 broken into a reibungsbreccia. Li some places the folding has been so 

 severe as to entirely destroy the thin belts of graywacke, producing out of 

 them lai'ge numbers of pebbles and bowlders. All stages of the transition are 

 found between the continuous belts of graywacke and the pseudo-conglom- 

 erate in the slate. 



