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- 
