CHAPTER IV. 



THE LOWER HURONIAN. 



SECTION L— SEDIMENTAEY ROCKS. 



OCCURRENCE AlfD SUBDIVISIOlSrS. 



The Lower Hm-oniau sediments of the Vermihoii district have a very- 

 large surface extent. They are present in two large detached areas — one, 

 known as the Vermilion Lake area, extending eastward from the western 

 limit of the area mapped near Tower, on Vermilion Lake, to within about 1 1 

 miles of Ely; the other, known as the Knife Lake area, beginning about 

 7 miles west of Ely and extending eastward to the eastern limit of the area 

 mapped. These same rocks extend farther eastward for an unknown but 

 great distance, passing north of and around the granite of Saganaga Lake 

 into Canada. Where the Vermilion and Knife Lake areas approach 

 each other — that is, west of Ely — the rocks have their least surficial extent, 

 rapidly widening as we follow them from this point eastward or westward. 

 This distribution is due to the fact that the sediments occur in two great 

 synclinoria. The short distance of about 5 miles by which the continuity 

 of the rocks is interrupted represents the place where, as a result of a 

 cross anticline, the lower (Archean) rocks have .been brought to the surface. 

 This gap is so narrow, and the structure points so clearly to the original 

 extension of the sediments across it, that this lack of continuity is not con- 

 sidered important. Clearly the rocks of the two areas were continuous 

 before erosion separated them. Considered in a broad way, the sediments 

 of the Lower Huronian are fragmental rocks consisting predominantly of 

 conglomerates and slates, althoug-h fragmentals intermediate between con- 

 glomerates and slates are, of course, present. 



It is difficult to estimate the relative quantity of the several kinds of 

 elastics included in the sediments. Moreover, they differ in respective 

 quantity in different parts of the area. The conglomerates form by far the 

 more striking portion, and the casual visitor to the district will notice 

 beautiful exposiires studded with brilliant-red jasper pebbles, and draw, 



