THE IRON-BEARING MEMBER. 295 



at an angle of G0° or 70", consisting of tlie ferruginous chert of the Iron 

 member itself These foot walls form the southern boundary of the ore 

 bodies. 



The formations are cut at right angles by a series of dikes, which are 

 in such a position that tlu^ same dike is generally at a greater depth as it is 

 followed eastward, i. e., pitches to the east. 



The ore bodies rest in the right-angled troughs formed by the junction 

 of the dike rocks and foot walls. 



The iron carbonate is the source of the ore deposits. 



The ores have not been chiefly ])roduced by the oxidation of the 

 carbonate in place, but 1)}- concentration from the lean carbonates of the 

 formation. 



The concentration has taken place dmnng or subsequent to the uplift- 

 ing of the series. 



Percolating" water was the active affent of concentration. It has 

 taken the carbonate into solution, and in its passage downward has been 

 deflected into the troughs ])y the impervious quai-tz-slate formation and the 

 dikes. 



In the apices of these troughs other waters more dii-cn^tly from the 

 surface, bearing oxygen, have precipitated the iron as an oxide. 



