170 The A'Dierlean GeolfKjixt. September, 1892 
:i. It is overlain by or results from cliange in taconyte. 
4. It is at or near the liori/on at wliicli tlie great gal)bro distiirbance is l)elieved 
to have occurred. 
5. It is associated with much kaolin or stratified sedimentary kaolinic rock. 
ti. At the same horizon is an important ore stratum the whole length of the range 
— at least 1.50 miles in Minnesota. 
T. In the near vicinity of the gabbro invasion the ore is hardened and largely con- 
verted to magnetite. 
8. It exists also as an important ore horizon further south, and beneath 480 feet of 
black slates, having been struck by diamond drill. Similar facts are reported 
from Menominee, .Mich. 
!). The whole ore bed is sometimes an original breccia, and at times some of the 
associated rock strata consist of coarse breccias and of conglomerates. 
10. No theory yet proposed for this ore is wholly acceptable. 
11. If the assiirnment of the date of the gabbro to this horizon be fully established, 
it would furnish a cauxa vera for some of the i)hysical features, and would sug- 
gest an intimate relation between the ore and the gabt)ro. 
Perhaps there is no more important or more interesting question, 
at present debated, relating to the iron ores of the Northwest, than 
that of their origin and their stratigraphic relations. From an eco- 
nomic standpoint, no less than from a scientific, there could Ite no 
more important question, for it is not until the geological relations 
and origin of these ores are understood that proper methods of min- 
ing them can be entered upon, and with the least expense. It is 
because of recent studies in the field, adding some new facts to 
the solution of this problem, or complicating it by the injection 
of some new conditions, that the writer desires to review the ele- 
ments of the problem and to show the difficulties that yet lie in 
the way. It will be well to enumerate briefly the hypotheses that 
have been propounded recently, as an introduction to this discus- 
sion. There are five. 
1. ,Siihstifi(fi(m for Itiiu'stoiic. Microscopic examination re- 
vealed the existence of remnants of calcite, or dolomite, in some 
of the "cherts" accompanying these ores in some places, and after 
long research the late professor Irving arrived at the conclusion 
that the whole body of the ore horizon was originally in the form 
of a limestone essentiall3',a "cherty carbonate." simulating in ori- 
gin and essential characters, the black-band ores of the Carboni- 
ferous. This inferentially led to the idea of a great primordial 
carboniferous age. 
2. Siil'stitntion for carhouatc of iron. Owing to the frequency 
of such a change observed in nature, it was but a short step to 
suggest a carbonate of iron instead of a carbonate of lime. This 
would more readily supply the iron, which must be explained, 
than carbonate of lime, and at the same time would only re([uire 
a slight change, if an}-, in the nature of the original rock, and in 
the conditions of its deposition. 
3. Concentration of iro)i o.ritJc from tin decay of ferrifuroua 
