GEOLOGY 243 



formed siliceous rocks. This mode of formation has been 

 described by Van Hise for similar ores in the Lake Superior 

 region. 



On the three southern islands of the chain there is a gradual 

 change in the nature of these measures. They pass into a 

 brownish-black siliceous shale, rich in iron and containing con- 

 siderable carbon as small scales of graphite. This is the form 

 in which they are found to the southward on the islands as far 

 as Long island. The thickness of the division is very constant 

 on the islands northward to McTavish, but it does not occur on 

 Cotter island. 



The rocks belonging to the third division, as before stated, 

 grade into the division above them, and the line between them 

 cannot be drawn sharply. 



The typical rock of the division is fine-grained and very 

 siliceous, with minute particles of silica coated with red oxide 

 of iron, forming a coarse impure red jasper. 



These jasper rocks usually occur in thin broken bands, the 

 partings between them being filled with a finely-divided mixture 

 of hematite, magnetite and jasper. The hematite is greatly in 

 excess of the magnetite. The association of the iron ores and 

 the jasper is intimate, and they must have been deposited simul- 

 taneously from aqueous solutions probably leached from the 

 cherty carbonate measures above. Microscopic sections from 

 these rocks are almost identical with those of jaspilite figured 

 by Van Hise in his monograph on the iron-bearing rocks of the 

 Lake Superior region ; and they must have had the same origin 

 that he has assigned to these rocks, namely, enrichments 

 deposited by water subsequent to the formation of the bedded 

 rocks in which they are found as partings, and filling the most 

 minute cavities. 



The amount of ore in this admixture of hematite and jasper 

 varies greatly. Where the ore is poor, the jaspery rock pre- 



