720 THE JOURNAL OF GEOLOGY. 
their original concentration in such places. The fact that the southern 
dipping monclinals are folded into a series of slight anticlines and 
synclines, combined with the presence of iron ore in basins, seems to 
ine to be strong evidence that many if not all the larger ore bodies in 
the Mesabi district as elsewhere were concentrated in pitching synclinal 
troughs. The recent development of the mines confirm this conclusion 
and give no evidence that the large ore deposits have formed at anti- 
clines or faults. The following laws, worked out in reference to the 
other districts of the Lake Superior region, appear also to apply to 
the Mesabi district, and if so, they may be said to be universal for this 
region. 
(1) The iron ores always rest upon a relatively impervious basement. 
This basement may be a shale, a slate, a quartzite, an amygdaloid, a 
volcanic tuff, an intrusive mass or a dike, a less porous layer of the 
iron-bearing formation, or any combination of these. (2) Large ore 
bodies are chiefly found where the impervious basements, simple 
or complex, form pitching troughs. (3) These pitching troughs are 
particularly likely to bear unusually large ore bodies when the iron- 
bearing formation has been brecciated or shattered by folding or some 
other process, so as to allow ready entrance to percolating waters. 
Within the troughs the iron-bearing and oxygen-bearing solutions 
have been converged and mingled, thus precipitating the iron oxide. 
Spurr,’ in 1894, discusses the stratigraphic position of the Thomp- 
son slates of northeastern Minnesota. ‘These have heretofore been 
correlated with the Animikie slates. However, almost every phase of 
slate of the Thompson series can be duplicated by the less altered 
phases of the Keewatin schists of the Mesabi range. In the vicinity of 
the Mississippi River the Thompson series becomes partly crystalline, 
being changed into sericitic, micaceous, hornblendic, staurolitic or gar- _ 
netiferous schists which correspond exactly with the green schists of 
the Keewatin. The cleavage of the Thompson series marks a distinc- 
tively pre-Animikie disturbance. The trend of the cleavage corre- 
sponds with that of the schistosity of the Keewatin of the Mesabi 
range, and it is thought that the two were developed at the same time. 
The Thompson series has undergone a considerable folding, and in 
this respect also more resembles the Keewatin than the Animikie slates, 
*The Stratigraphic Position of the Thompson Slates, by J. E. SpuRR. Am. 
Jour. Sci., Vol. XLVIIL., No. 294, pp. 159-165, August 1894. 
q 
t 
Tie... 
