THE REPUBLIC TROUGH. 551 



usually normal to the strike of the quartzite. Tliese deposits occur in the 

 arches or on the limbs of the minor contortions, and never, so far as 

 observed, occu])y the troughs. Their attitude witli reference to the general 

 sti'ike of the (juartzite, and the fact that they do not show the contortions 

 of the inclosing specular jasper, prove that they have come into existence 

 since the folding. 



ORIGIN OF THE ORE DEPOSITS. 



From the form and general relations of tlie rich ore deposits it is 

 evident that they were not laid down as bodies of rich ore contemporaneously 

 with the inclosing rocks. It is not conceivable that nearly pure silica and 

 nearly pure iron oxides could be deposited under water at the same time on 

 opposite sides of an imaginary vertical plane. Nor is it any more probable 

 that they have come up from below as igneous dikes which have intruded 

 the sediments of the iron formation. The physical objections alone to this 

 view are such as entirely to exclude it from serious consideration. On the 

 other hand, the phenomena of their relations to the inclosing rocks, which 

 have been described, all lead to the conclusion that they are later concen- 

 trations and indicate the main lines along which the concentration was 

 brought about. 



In general, this process of concentration has been a removal by cir- 

 culating waters, in favorable places, of the silica of the old rock, and its 

 contemporaneous replacement by iron oxides. This process has gone on in 

 the contact zone, in the detrital conglomerates, and in the underlying 

 jaspers. The evidence in both cases is abundant and clear. In the case of 

 the iron formation of the lower series the siliceous bands may be traced 

 along the strike in all stages of replacement, until finally they are wholly 

 represented by new iron oxides. In some cases the new iron ore is of coarser 

 texture than the old, and so the original banded structure may still be 

 traceable into a bod}- of nearly pure ore. In the case of the conglomerates, 

 we see in thin sections original rolled quartz pebbles, which are sometimes 

 surrounded by new gi'owths of quartz, studded with iron oxides about the 

 periphery. This process, too, may be traced through all stages, from cases 

 in which the attack on the old pebble had just begun to those in which 



