Iron Ores of Minnesota.—IVinchell. 159 
silica, but which still contains both substances. The transi- 
tion to this rock is not always abrupt, but sometimes it is quite 
gradual, there being a gradation or alteration from the rock to 
the ore. Underlying the ore horizon is almost always a-sand- 
stone or quartzyte, although this is wanting at the eastern end 
of the range and the ore comes directly on the granite or 
greenstone of the Archean. Overlying the ore is a black slate, 
and this black slate is also somewhat inter-stratified in the ore 
at a few points. This black slate becomes more siliceous and 
coarse, making quartzyte, and develops into a great thickness. 
Unconformably over the whole country the Cretaceous ocean 
deposited its own sediments, but these have as yet been found 
only in isolated places, and they present no obstruction to the 
prospector or the miner. The drift deposits are heavy and 
reach in some places a thickness of a hundred feet. In the 
productive part of the range the iron bearing rock, and the 
ore, are wholly hid by the drift sheet. 
The most interesting points in the natural history of the 
iron ores of Minnesota are connected with their origin. Iron 
ore, like all ores, has had a cause for its existence, some cause 
however, inherent in the operations cf nature which has pro- 
moted its accumulation at certain places in greater amount; 
for all the ores, and especially iron ore, are widely disseminat- 
ed. There is probably not an ounce of natural water on the 
face of the earth, unless it be freshly fallen from the clouds, 
that does not contain a small amount of iron in some form. 
The problem has been to learn the factors that have collected 
this iron in large amounts at certain places. 
The late R. D. Irving supposed it to have resulted from the 
oxidation of a carbonate of iron. He postulated, therefore, a 
great primordial vegetable age whose characteristics could be 
compared to those of the Carboniferous, and whose function was 
to store up carbon, and secondarily iron ore. Carbon and iron 
ore are frequently associated, as in the Coal Measures, the fo-- 
mer taking the chemical combinations of limestone and the 
latter of kidney iron ore. In the application of this theory 
the kidney iron ore, and the siliceous carbonate of lime are 
supposed to have combined to produce a “cherty carbonate,” 
and from this last the present ores resulted from simple ox- 
idation and concentration. The fortuitous positions of the 
