62 THE CRYSTAL FALLS lEOK-BEAEING DISTRICT. 



The strono- similarity between the composition of these clay slates is 

 at once apparent, and needs no further comment. The only marked differ- 

 ence between the Huronian clay slate and the Cambrian ones is the higher 

 percentage of alumina present in the former. 



SIDERITE-SLATE, CHERT, FERRUGINOUS CHERT, AND IRON ORES. 



The two most interesting kinds of rock from the Mansfield slate belt 

 are those known as the siderite- or sideritic slates and the cherts or ferrugin- 

 ous cherts, according to the quantity of iron carbonate and iron oxide 

 present. These alternate ^vith each other, and are found also interstratified 

 with the fragmental slates, and thus there can be no question as to their 

 sedimei^tary character. The siderite-slates are of a light to dark gray color. 

 They are well laminated, and in some places cleave rather readily along 

 the laminae, though at other places they break with an almost conchoidal 

 fracture. The weathered siderite slates are covered by a crust of reddish- 

 brown hydi'ated iron sesquioxide. 



Microscopically the siderite slates are composed of siderite, or of sider- 

 ite and exceedingly fine grained cherty silica. Roundish rhombohedra of 

 siderite compose the purer sideritic portions. If one passes from the pure 

 to the less pure slates, the siderite gradually diminishes in quantity, the 

 silica grains increase correspondingly, and the rock grades into the chert 

 which, in bands, is commonly associated with iron carbonate in the Lake 

 Superior region. As the carbonate alters to the oxide or hydrated oxide 

 ferruginous cherts are produced. The cherts are white to red, depending 

 on the amount of iron oxide present. The manner in which the siderite 

 alters to limonite and hematite, and the various steps of the process have 

 been so well described and beautifully illustrated in Monograph XXVIII, 

 that the reader is referred to that volume for further information. None of 

 the brilliant red jasper or jaspilite, such as that found in the Marquette 

 district, is associated with the Mansfield slates. Iron ores of economic 

 importance, however, are found associated with these slates, and are 

 described in detail farther on. None of the sideritic slates, ferruginous 

 cherts, or ores, although interbedded with the fragmental slates, show any 

 evidence of fragmental origin so far as the indi^^dual grains of the minerals 

 composing them are concerned. 



