440 THE VERMILION IRON-BEARINa DISTRICT. 



Soudan formation to fen-uginous slates and ferruginous cherts, although 

 residual iron carbonate undoubtedly remained. 



It therefore appears that the batholithic and dike intrusions of granite, 

 porphyry, etc., and doubtless extrusions, and the orogenic and profound 

 erosion and the metamorphism were contemporary events, or at least largely 

 overlapping. While the intrusive masses are placed in the Archean series, 

 these events chiefly mark the inter- Archeaii-Huronian time, the most con- 

 spicuous evidence of which is the great unconformity between the Archean 

 and the Lower Huroniau series. It is clear, however, that Archean time is 

 not sharply separated from inter-Archean-Hurouian time, but is connected 

 through the intrusive masses. If there were volcanics contemporaneous 

 with the batholiths of granite and the intrusive masses of porphyry, these 

 were wholly removed by the great period of erosion mentioned below, since 

 no such rocks are found in the district. 



After erosion had long continued either the laud was reduced to the 

 level of the sea by this process or else subsidence came, or perhaps erosion 

 and subsidence combined to reduce the outer portion of the Vermilion 

 district to the level of the sea. The sea then encroached upon the land. 

 However, the land was uneven, with highlands here and lowlands there, so 

 that the time of the advance of the sea over the district varied considerably. 

 The areas first encroached upon received a thick layer of gravel and 

 bowlders, the material being derived from highlands which had not yet 

 been covered by the sea. Thus there was built up the great Ogishke con- 

 glomerate, the lowest formation of the Lower Huroniau series. When at 

 last the sea had succeeded in entirely overriding the district, the conditions 

 were no longer favorable for the deposition of coarse mechanical sediments; 

 also it is probable that a certain amount of subsidence further favored 

 quiescent conditions at the bottom of the sea. At this time the Agawa 

 formation was laid down in certain favorable localities, but in small quan- 

 tity, in the eastern half of the district. 



In the western part of the district no chemical or mechanical deposit 

 was found which may be correlated with the Agawa formation in the 

 eastern half of the district. It may be supposed that here the water was 

 not deep enough for such sediments to be produced. 



Following the deposition of the rocks of the Agawa formation must 

 have been a long-continued subsidence, for upon the formation was laid 



