224 THE MARQUETTE IROXl'.EARING DISTRICT. 



The slates and <>Tay\vaekes usually pass quickly into the typical 

 quartzite of the formation. Within the Mesnard quartzite is an interstrati- 

 fied cong-lonierate, from a few inches to 40 feet in thickness, in which are 

 abundant frag-ments of ferruginous schist, of quartz, of chert, and of jasper. 



The ([uartzite is in general a rather pure vitreous quartzite, very mas- 

 sive in hand specimens, but in the ledges often showing distinctly the 

 bedding, and not infrequently passing into slaty phases. In many places 

 at the east end of the Mesnard i-ange the original ripple-marked surfaces of 

 the layers are observed. The intricate windings of the conglomeratic chert 

 and jasper pebble-bearing layer were traced out, and were of great assistance 

 in determining the structure. Where the folding has been close the quartzite 

 passes into a very vitreous rock, or even into a (juartz-schist. The vitreous 

 rock is produced bv extensive fracturing, or even brecciation, and the filling 

 of the resultant minute and large cracks with vein chert or quartz. The 

 veins vary from those of minute size to those several inches across, and in 

 some cases they anastomose through the quartzite in every direction. This 

 secondary material often closeh* resembles the original stained or g]-anu- 

 lated quartz gi-ains, but the rooks as a whole take on a j)eculiar aspect, and 

 have been called cherty quai'tzites. 



At the top th(i quartzite ])asses into slaty phases, and these grade into 

 slate, a belt of which, from less than 30 to 100 feet thick, separates the 

 quartzite from the Kona dolomite. The Mesnard quartzite may then be 

 divided into foui- members: (1) Conglomerate, (2) slate and graywacke, 

 (3) qup'tzite, and (4) slate. The quartzite is the predominant member. 

 Slates and graywackes are locally intermingled with the quartzites. A sin- 

 gle section showing all tlie phases is rarely found, and exposures are not 

 sufficiently numerous to enable one to make these subdivisions in mapping. 



Microscopical. — Tlie conglomeratcs are of tlu'ee main kinds: (1) Those 

 adjacent to the Mona schist: (2) those adjacent to the granites; and (3) 

 those interstratified witli the graywacke or quartzite. The first occurs along 

 the northern border of the Algonkian, the second along the southern border, 

 and the third at various jjlaces along lioth the northern and southern belts. 



(1) The northern conglomerate is in its lower parts a stucco of granite 

 and green-schist fragments set in a sparse matrix. The granitic pebbles and 



