186 THE VERMILION IRON-BEARING DISTRICT. 



surrounding cherts. A more or less perfect false bedding observed in 

 some of these cherts (PI. V, B) might also be considered the result of 

 water motion. But this would not prove the cherts to be of mechanical 

 origin, as we find this structure in sediments of oj'ganic origin also. The 

 fine grains of quartz constituting the chert contain scattered through them 

 minute crystals and specks of magnetite and hematite and areas of limonite. 

 In some places these crystals are very small — mere dust as it were; in others 

 they are of considerable size. At some places the quartz grains will con- 

 tain very few of these diTst specks; at others they are nearly full of them 

 and appear almost opaque. The crystals of the iron oxides may occur at 

 any and all places in these quartz grains, from the center to the periphery, 

 and when the crystals are large they not infrequently extend from one 

 quartz grain to another, running across the junction of the grains. In some 

 cases these polygonal grains are outlined very distinctly by iron oxide, 

 occurring either as a mere film or as a layer of considerable thickness. 

 Instances were observed where the grains of quartz in the chert were 

 heavily impregnated with particles of iron ore on the periphery, leaving but 

 a small, fairly clear center. Other instances were observed where there was 

 less of the clear central quartz present, and, in fact, there seemed to be all 

 gradations from these cases up to those in which there was an opaque mass 

 of ore giving but an occasional indication of the presence of quartz. 

 These facts seem to show that the ore in these rocks is not primary, but 

 is a secondary product which has been accumulated either in bands or in 

 irregular masses as the result of the replacement of silica by iron oxide. 



There is one vai'iety of the chert which is interesting, for it seems to 

 give a clue to the siliceous rock which has been replaced by the ore. This 

 variety is the greenish carbonate-bearing chert to which reference has 

 already been made. Under the microscope such cherts show up as finely 

 granular aggregates of silica in normal rounded polygonal grains, but asso- 

 ciated with the silica grains is a carbonate which occurs iu rounded rhombo- 

 hedra. The rounding of these grains is not the result of meclaanical action. 

 A study of the slides shows the alteration of the carbonate to limonite. A 

 further change, resulting from dehydration, would produce a hematite- 

 bearing chert, and in cases where the oxidation of the carbonate took 

 place with access of insufficient amount of oxygen there would be produced 

 magnetitic chert. In rocks containing a large proportion of carbonate and 



