266 THE MARQUETTE IRON-BEAlJUsTG DISTRICT. 



By a decrease in the size of the coarser fragmental grains the gray- 

 wackes pass into the slates. In these slates the decomposition of the feld- 

 spar grains, because of their smaller size, is much more common. On 

 account of the more plastic character of the slates, there is frequently 

 develojied in them a slatv cleavage or schistose structure, the ordinary 

 cleaved slates passing into mica-slates, and occasionally into sericite-schists. 

 In passing from the less mashed to the most mashed phases there is an 

 increase in the regularity of the arrangement of the sericite leaflets in a 

 imiform direction. As in tlie graywackes, the rocks are usually impreg- 

 nated to a greater or less degree by iron oxide, and frequently very 

 heavily so. The iron oxide includes limonite, hematite, and magnetite, the 

 two latter often being in large part in Avell-defined crystals, and sometimes 

 in veins. Frequeuth' the slates consist of layers of differing degrees of 

 coarseness, sometimes a half dozen fine and coarse laminjB being observed 

 in a single section. In these cases the coarser bands are more likely to be 

 heavily iron-stained, the accommodations apparently having formed cracks 

 and crevices to a greater degree than in the iuterlamiuated finer and jnore 

 plastic laj-ers. 



The slates and graA'wackes at times become conglomeratic, so that 

 whole ex])osures are slate-conglomerate, or else the conglomerate layers 

 are interstratified with the ordinary slate and graywacke. These slate- 

 conglomerates bear exactlv the same relation to the slates and gra^'wackes 

 that the basal conglomerate does to the quai-fzite — that is, there are pebbles 

 and bowlders in the slate or graywacke background. These pebbles and 

 bowlders are identical in lithological character with those of the basal 

 conglomerate, l)ut, upon the whole, they are better rounded. In certain 

 places the later movements which these slate-conglomerates have imder- 

 gone have brecciated them, so that with the water-rounded fragments are 

 apparent ])ebl)les of slate and graywacke. A close examination of these 

 in the field, and especially in thin section, shows that they have angular 

 forms and are clearly produced by the breceiation of the rock itself. This 

 occurrence was pai'ticularlv confusing, as the rock is an undoubted con- 

 glomerate, and yet a conglomerate which is })artly autoclastic. 



