132 THE MAItQl'ETTE IltOX BEARIXG DISTRICT. 



peuetratiii;j;- dikes or h\ offshoots from large masses of "diorite" the ore 

 deposits are the thickest. 



Large ore bodies of the fourth class are unknown. 



All the ore Ijodies of the second, third, and fourth classes lie wholly 

 within tlie lower iron formation, and at any horizon within it, provided in 

 the second and third cases there be present a "soapstone." Thus the distri- 

 bution of the ores is similar to their distribution in the Penokee district, 

 where thev have been shown to l)e dependent upon the presence near 

 them of some rock impervious to water. 



All tlie facts bear toward the couclusiou that the ore i.s a secoudary concen- 

 tration produced by the action of downward percolating water. * * * The ore 

 deposits occur at places where circulating waters are sure to be concentrated. The 

 soap rock accommodates itself to folding without fracture, and while probably allowing 

 more or less water to pass through, acts as a practically impervious stratum, along 

 which water is deflected when it once comes in contact witli it. * * * On the other 

 hand, the brittle, siliceous, ore-bearing formation has been fractured by the folding to 

 which it has been subjected, so that -yhere these processes have been extreme water 

 passes through it like a sieve. That the tilted bodies of diorite or soap rock, especially 

 when in a pitching synclinal or forming a pitching trough by the union of a dike and 

 a mass of diorite, must have guided dowuward-tlowing waters is self-evident. * * * 

 It is also idain that the contact plane between the quartzite-conglomerate and the 

 ore-bearing formation, that is, the plane of unconformity between the Upper and 

 the Lower Marquette series, must have been a great horizon for downward-flowing 

 waters. (P. 125.) 



Alou"- these channels silica was removed and iron oxide concentrated. 



After aro-uino- the question the author concludes that the concentration 

 of the ores occurred, in all probability, "during and later than the folding 

 and erosion subsequent to Upper Marquette time" (p. 126),thi-ough the long 

 continued action of percolating waters, flowing downward along channels 

 whose courses were determined by the existence of imper^^ous rock masses 

 cuttino- tlu-ouo-h less impervious ones, or were directed l)y the contact plane 

 between the Upper and the Lower series. The iron probably came from the 

 formation in which the ore now occurs, and the diabases in their alteration 

 to "soapstones" provided some of the alkalis necessary to dissolve the silica. 



The difference between the soft and the hard ores is supposed to be 

 due to the fact that the latter were first deposited in a crystalline condition 



