48 The American Geologist. January ee 
This paper has three chapters, the first of which contains 
a general discussion of principles. Here the stratigraphy of 
the region is outlined, and the iron-bearing formations in par- 
ticular are described. There are three rock series which con- 
tain iron horizons of importance—the Archean, the Lower Hu- 
ronian and the Upper Huronian—in each of which practically 
the same conditions favorable for the production of large ore 
bodies have existed. The general process of ore formation for 
the whole Lake Superior region is the same as that already 
described in the monographs on the Penokee-Gogebic and the 
Marquette districts. In brief this consists of (1) the leaching 
of the iron from older, mainly igneous, rocks and its deposition 
in a largely non-clastic sedimentary formation; (2) the rocks 
of this iron-bearing formation were originally, or have become, 
cherty carbonates; (3) circulating meteoric waters have dis- 
solved and carried downward this iron carbonate, and the iron 
has been précipitated as an oxide; (4) along with this precipi- 
tation and the consequent enrichment of favorable parts of the 
iron-bearing formation, replacement has also taken place, the 
siliceous part of the rocks having been removed; (5) the ore 
bodies thus formed occur in pitching troughs, the bottom and 
sides of the troughs being composed of rather impervious rock. 
The ore bodies are thus formed in and from the iron-bearing 
formation by descending waters. In this paper it is stated 
that the original rock of the iron-bearing formation, instead 
of always being a cherty carbonate, may have had the iron in 
part in another form, such as a sulphide (pyrite) or a silicate. 
The changes outlined in the last paragraph are the normal 
changes taking place in the iron-bearing formation near the 
surface. These changes result in the production of ore bod- 
ies and the peculiar rocks which accompany them, such as jas- 
pilytes, ferruginous cherts, etc. In some places, however, this 
formation has been subjected to deep-seated changes, some- 
times accompanied by contact metamorphism, and here amphi- 
bole-quartz-magnetite rocks, and in extreme cases pyroxene- 
olivine-quartz-magnetic rocks, have been produced. In such 
places no ore bodies of economic importance are known. 
The second chapter is devoted to a description of the ore 
bodies and the formations in which they occur in the different 
districts. There are six .of these districts—the Penokee-Go- 
