OKIGIN OF IRON ORES OF NEGAUNEE FORMATION. 401 



that the couditious governing their formation are mucli tlie same. In both 

 districts the material immediately underlying the ore is relatively impervi- 

 ous to water. In the cases of the deposits resting upon soaprock this lack 

 of porosity is nearly complete. Most of the ore bodies are in troughs in 

 both districts; the ore bodies in both, in longitudinal section, have a pitch; 

 in both the many phases of material found in the iron 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 a 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 e^-ident 

 that they are not eruptives. (Pis. XXVIII and XXIX.) No eruptive 

 would be found in such strange shapes and relations. It is equally certain 

 that these irregular masses of ore are not altogether fonned by direct 

 sedimentation, althougli a considerable part of the iron oxide in an ore body 

 may be an oxidation ))roduct in place of a sedimentary iron carbonate. 



All these facts bear toward the conclusion that the ore was secondarily 

 enriched by the action of downward-percolating water, since the ore deposits 

 occur at places where percolating waters are sure to have been concentrated 

 The soaprock accommodated itself to folding without fracture, and, while 

 probably allowing more or less water to pass through, acted as a practically 

 impervious stratum along which water was deflected when it came in con- 

 tact with it. It is a common opinion among miners that a few inches of 

 soaprock 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 was fractured by the folding to which it was subjected, so that 

 where this process was extreme water passes through it like a sieve. That 

 the tilted bodies of greenstone or soaprock, especiallv when in pitching 

 synclines or forming- pitching troughs by the union of dikes and masses of 

 greenstone, must have converged downward-flowing waters is self-e\ddent. 

 It is also clear that the weak contact plane lietween the Goodrich quartzite 

 and the Negaunee formation was one of accommodation and shattering 



MON XXVIII 26 



