PUBLICATIONS. 77 
the width of the formation varying correspondingly from two or three 
miles to less than half a mile, the average width being one mile, and 
the average dip 10°. Resting upon the iron-bearing member is a 
great thickness of fine-grained slates, at the base of which is locally 
an impure dolomitic limestone. When this limestone is present, the 
contact between the iron-bearing member and upper slate cannot be 
distinctly located. 
The least altered phase of the iron-bearing member is a rock called 
taconyte, which consists of a background. of cryptocrystalline, pheno- 
crystalline and chalcedonic silica, in which are numerous granules. These 
are composed of glauconite, siderite, hematite, magnetite, limonite and 
cryptocrystalline silica, in the very freshest phase the two former being 
predominant. ‘The granules in one of these fresher phases by analyses 
showed about 35 per cent. of siderite and 65 per cent. of glauconite, or 
about 22 per cent. of ferrous oxide in the form of siderite; and about 
Io per cent. of ferrous and ferric oxide, two-thirds being the former 
in the glauconite. Other analyses gave similar results. Analyses 
showed a very little calcium and magnesium. In the freshest phase 
found were seen, in thin section, probably detrital original grains of 
carbonate, recognized by their cleavage as calcite or dolomite. From 
the taconyte, by a complicated series of metasomatic changes, there 
have developed cherts and jaspers, which are sideritic, hematitic, 
magnetitic or actinolitic, or two or more of these combined. During 
the process the chert and iron oxides were largely concentrated in 
alternating bands. The cherts and jaspers are frequently concretionary 
and brecciated. ‘They have often a prismatic jointing and horizontal 
parting. 
These transformations were caused by downward percolating waters, 
carrying as the chief agents oxygen and carbonic acid, and as sub- 
ordinate agents sulphuric acid and alkalies. In the changes from 
glauconite and siderite to the oxides, there was an important shrinkage 
of the mass, and this has resulted in the brecciation, prismatic jointing 
horizontal parting and banding. ‘The prismatic jointing is analogous 
in its formation to the shrinkage of basaltic columns of lava. The 
horizontal parting is caused by a later shrinkage along the least 
diameters of the columns formed by the prismatic jointing. The 
banding is due to the removal of silica and the entrance of iron along 
the parting. 
The ore deposits rests upon the Pewabic quartzite, or upon the 
