THE IKON BEARING MEMBER. 291 



u'on formation. Here the relations of the ore to the dikes and all other 

 of its characters are the same as when the ore is found upon the fragmental 

 quartzite. It is of interest here to note, that in one of the largest mines of 

 the district, the Colby, as shown by PI. xxxi. Fig. 3, the north and south 

 deposits have as their basement the same great dike, the two ore bodies 

 being separated at the to}) merely Ijy a gigantic horse of rock, which served 

 as the impervious layer to form the foot wall of the north deposit. The ex- 

 planation given of the origin of the ore found upon the fragmental quartzites 

 applies perfectly to these north deposits with the modifications above indi- 

 cated. That there are layers of the iron formation which are not readily 

 pervious, and therefore become basements along which the down-flowing 

 waters passed, is not at all strange. It would be stranger if, in a thickness 

 of water-deposited sediments of 800 feet, there were no layers which, at 

 least for a short distance, were effectual barriers to the passage of percolat- 

 ing waters. The chemistry of the process of concentration of the ore 

 deposits east of Sunday lake is, in all probability, like that of the typical 

 deposits of the range. Their concentration is apparently, however, more 

 nearly analogous to the narrow seams of ore described in the early part of 

 this section than to the typical deposits. The formation here apparently 

 being cut by no impervious dikes, the waters have not been carried over to 

 the quartzite, thus forming main channels of percolation, but the compara- 

 tively small ore bodies have developed here and thero as favoraljle C(indi- 

 tions for concentration occurred.' 



The al)ove explanation of the origin of the ore deposits accords well 

 with the facts of their occurrence, and also with the idea that the iron 

 formation was originally an impure cherty carbonate of iron. It explains 

 perfectly tlie peculiar position of the ore bodies with reference to the dikes 

 and the foot- wall ([uartzite ; it explains their presence in a similar position 

 in the few instances in which the deposits are uortli of tlie fragmental 

 quartzite ; it explains the flat wedge-shaped character of the ore deposit ; it 

 explains the nature of the ore — a soft somewhat hydi-ated hematite, bearing 



' As boariug upon tlm truthfiiln&ss of the iibovo theory as a whole, it is .an iutoresting fact that 

 the practical miuers, in prospecting, eagerly follow underground water channels, hoping that they 

 will lead to ore deposits. 



