336 The American Geologist. May, 1894 
General Structure, 
The lowest rocks of the region are greenish schists, which 
belong in the Keewatin formation.* These schists are cut by 
a great belt of intrusive granite, which runs the entire length 
of the iron-bearing district and usually forms the summit of 
the divide betweeen the Mississippi and the Red river basins. 
Unconformably upon these older rocks lie the gently dipping 
Animikie strata. 
There are three chief members of the Animikie in the iron- 
bearing regions, as definitely known at present. Lowest is a 
quartzyte; upon this lie the iron-bearing rocks ; and finally 
there is a great thickness of black slates. The base of the 
slates is calcareous, and becomes in places an impure lime- 
stone, often dolomitized or sideritized. 
The iron-bearing rocks occup} r a definite and constant hori- 
zon between the quartzyte and the slates. They are marked 
by peculiar and characteristic features, and have always been 
recognized as invariably associated with the ore deposits. 
They seem to have a nearly uniform thickness, which may be 
estimated as between 500 and 1,000 feet, with an average of 
perhaps 800 feet. The relationship of these several forma- 
tions is shown by the section forming figure 1. plate vm. 
Minok Structure ok the Animikie. 
The ore-bearing region of the Western Mesabi affords a 
peculiarly valuable field for investigation, for the rocks have 
suffered only very slight disturbance since the time of their 
deposition. The general structure is a monocline, which dips 
slightly east of south, at a gentle angle, which averages per- 
haps ten to fifteen degrees. On the eastern end of the West- 
ern Mesabi there has been some slight additional disturbance. 
There is evidence leading to the belief that a wedge-shaped 
area, eight or ten miles in length, lying mainly in T. 5S-17. 
has been faulted up above the surrounding rocks; the amount 
of vertical displacement being perhaps 500 feet. This may 
be called the Virginia area, from the town of that name which 
* The terms Keewatin and Animikie are -used bj the Minnesota Sur- 
vey io distinguish formations which, in general, correspond to the 
Eluronian of the United Slates Geological Survey, the Animikie corre- 
sponding to the Upper, tile Keewnlili to tile lower Huronian. (Sec 
Twenty-firsl Ann. Rep., Minn. Survey, p. I.) 
