406 THE MAKQUETTE IKOX-BEARIXG DISTRICT. 



upon a relatively impervious basement. This may be a shale, a slate, a 

 greenstone mass, a dike, or two or more of these combined. Adjacent to 

 the ores all of these formations are apt to ])e modified and impregnated 

 with iron oxide, and are hence called soapstone or paint-rock. The large 

 ore bodies are found only when the impervious basements are in the forms 

 of pitching troughs. These pitching troughs are particidarly likely to 

 bear unusually large ore bodies when the iron-bearing formation is nmcli 

 shattered by folding. 



Ill iirospecting for the first class of ores, those that rest upon the Siamo 

 slate, a trough in the slate should be sought. A plunging synclinal trough 

 may be formed by a swing of the boundary line between this formation and 

 the iron-bearing formation; or a trough may be formed by a combination 

 of the slate with a cutting dike or mass of greenstone ; or a trough in the 

 slate may be supplemented by an intersecting greenstone. 



In the second class of deposits — those within the formation — the 

 pitching troughs are wholly formed by the intrusives. Here valleys of 

 the iron-bearing formation, when nearly surrounded by an amphitheater of 

 greenstone, furnish a particularly favorable area. Where the iron-bearing 

 formation in the A-alle\' is the ferruginous chert, rather than the griinerite- 

 magnetite-schist, the conditions are more favorable. Pitching troughs 

 bottomed by soapstone may exist underground which can not be discovered 

 at the surface, since, where an intersecting intrusive is of small size and 

 has been transformed to soapstone, it is eroded as rapidly as the iron forma- 

 tion, and thus its existence is not discovered by outcrop or any topographic 

 feature. 



The third class of deposits, the hard ores, must always be prospected 

 for near the contact of the Negaunee iron formation and the Goodrich 

 quaitzite. As in the previous cases, the ore bodies are particularly likely 

 to exist if the two are folded so that the contact forms a pitching trough, and 

 if this be bottomed by soapstone the conditions are still more favorable for 

 the formation of large deposits. 



The general map (Atlas Sheet IV) shows several extensions of the iron- 

 bearing formation which have not been ^^rospected. The arm running east 

 of Palmer has been prospected along its south side, but as yet almost no 

 work has been done along the north side. The exposures here are not 

 sufficient to indicate the minor bends of the iron-bearing formation, but 



