THE LOWER HURONIAN. 279 



Tower and spread out southward into a broad area in whicli exposures 

 are so rare that no structural details can be determined. It is presumed, 

 however, that this is also a great synclinorium, the south liml) of whicli 

 is bounded hj the Giants Eange granite, here beyond the limits of the 

 map. 



Within the Vermilion Lake area and immediately adjacent to it ex- 

 posures are sufficient to enable us to determine some of the details of the 

 structural features of the sediments in them. 



■ In this part of the Vermilion district it will be noted that the conglom- 

 erate lies upon the flanks of the anticlinal hills formed by the older under- 

 lying rocks. Within the area covered by the Lower Huronian sediments 

 alone, the Ogislike conglomerate occupies the anticlines, for example, at the 

 Pike Bay oval, whicli is an anticlinal area. Ely Island is made up chiefly 

 of the Ogishke conglomerate, but enough of the adjacent rocks are exposed 

 to show plainly the structure. The conglomerate occupies the main central 

 portion of the island — in fact, nearly all of the western two-thirds of the 

 island — with but a small area of the Knife Lake slates flanking it on the 

 south. In the eastern part of the island the conglomerate is intermixed with 

 eruptive rocks, the granites of Vermilion Lake, from which it is derived and 

 with which it is intricately infolded. On the eastern as well as on the 

 westei'n end of the island the conglomerates are coarser near the center, and 

 grow finer and finer toward the sides. This change is most noticeable on 

 the south side of the island, where at several places along the shore the Knife 

 Lake slates grade into the conglomerate through graywackes of intermediate 

 grain. 



In general the slates occur in synclines lying between anticlines of 

 older and harder rocks, and ordinarily these synclines coincide with the 

 topographic depressions. In some places, however — as, for instance, north 

 of Tower, between the west end of Soudan Hill and the point between 

 Swede and Middle bays — the slates occupy a minor synclinorium and are 

 extremely plicated. The slates of this particular synclinorium occupy at 

 this place higher ground than the adjacent conglomerate on its flanks, 

 and within this synclinorium the anticlines of slates are the structural 

 features that occur at the greatest elevation. Structural details, such as 

 strike and dip, were observed almost exclusively on the slates, and it is 

 consequently by a study of the slate exposures chiefly, assisted by observa- 



