184 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



this (like, as it occurs in a pitching trough, formed l)v its junctiou with the 

 impervious slate. Tliese same relations are well known to be the cause of 

 simihir occurrences in the Lake Superior districts above mentioned. 



The original rock from which the ores Avere formed was cherty iron 

 carbonate, which in many places is found associated with the iron-bearing 

 formation. The cherty carbonate shows the A-arious stag-es of alteration 

 from the compact cherty siderite to the banded ore and chert rocks which 

 form the nuclei for the addition of the iron obtained from the higher exten- 

 sions of the beds. Percolating waters have been the agents in this process 

 of replacement and concentration. Consequently where the rocks have been 

 most shattered, we iind the water was especially active. Hence it is, also, 

 that we find the deposits in this closely folded part of the Upper Huroniau. 



As to the origin of the cherty carbonate itself, we know nothing 

 definite. Its association with the carbonaceous slates would indicate the 

 agency of organic matter in its production, possibh' in some such manner 

 as is rather generally accepted for the formation of the Carboniferous 

 carbonate ores. The Upper Huronian ores, as well as the Lower Huronian, 

 are supposed to have been formed in this same way, and from the same 

 kind of rock. Under the discussion of the Lower Huronian ores (p. 70) 

 these points were discussed more in detail, and references given to the 

 literature, and the reader is referred to that discussion for further details. 



SIZE OF THE ORE BODIES. 



No definite general statement can be made as to the size of the ore 

 bodies, as this varies considerably. None of the bodies which are being 

 worked, so far as I can learn, are less than 30 feet wide. In one of the 

 old mines crosscuts disclosed a width of nearly 200 feet. This same ore 

 body is reported to be at least one-fourth of a mile long. 



METHODS OF MINING. 



The first development of the iron ores of this district was by the 

 stripping and. open-cut method, very few resorting at once to under-ground 

 work. Nearly two-thirds of the product of certain of the mines has been 

 from open-pit work. "When the open pits become too deep to be readily 

 worked as such, shafts are sunk and the exploiting of the ore body is 

 carried on under ground, at times both open-pit and under-ground work 

 being carried on simultaneously. The Mastodon presented the unusual 



