C. R. Van llise — Iron Ores of Michigan. 125 



material immediately underlying the ore is relatively im- 

 pervious to water. In the cases of these deposits which rest 

 upon soap-rock this lack of porosity is nearly complete. Many 

 of the ore-bodies are in troughs in the Lower Marquette series, 

 as are nearly all in the Penokee district. The ore-bodies in 

 both in longitudinal section have a pitch. In both the many 

 phases of material found in the ore-bearing formation are 

 nearly the same, and in both is found plentiful residual iron 

 carbonate. It is therefore thought that the explanation of the 

 origin of the ores in the Penokee district is applicable, with 

 few modifications, to those of the Marquette district, although 

 the larger number of the deposits of the latter belong to an 

 older series. 



The forms, attitudes and relations of the ore- deposits render 

 it evident that they are not eruptives. No eruptive would be 

 found in such strange shapes and relations. It is equally cer- 

 tain that these irregular masses of ore are not produced 

 directly by sedimentation. All the facts bear toward the 

 conclusion that the ore is a secondary concentration produced 

 by the action of downward percolating water. When the 

 facts are examined in detail, it is seen that the ore-deposits 

 occur at places where circulating waters are sure to be concen- 

 trated. 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 with it. 

 It is a common opinion among miners that a few inches of 

 soap-rock is more effective in keeping out water than many 

 feet of the iron-bearing formation. On the other hand, the 

 brittle siliceous ore-bearing formation has been fractured by 

 the folding to which it has been subjected so that where these 

 processes have been extreme water passes through it like a 

 sieve. That the tilted bodies of diorite or soap-rock, espe- 

 cially when in a pitching synclinal or forming a pitching 

 trough by the union of a dike and a mass of diorite, must 

 have guided downward -flowing waters is self-evident. The 

 pitch common to other deposits is also evidence in these 

 cases of laterally deflected downward-moving waters and indi- 

 cates a wholly or partly impervious substratum even when its 

 character has not yet been ascertained. It is also plain that 

 the contact plane between the quartzite-conglomerate and the 

 ore-bearing formation, that is, the plane of unconformity be- 

 tween the Upper and Lower Marquette series, must have been 

 a great horizon for downward-flowing waters. 



If it is true that the whole of the iron-bearing formation 

 was originally a lean cherty carbonate of iron, with perhaps 

 some calcium, and magnesium, or if we go no farther back 

 than the ferruginous cherts and jaspers, it is then concluded 



