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PROCEEDINGS OF THE WASHINGTON MEETING 



hard trap on the other, would shear off the top and cause it to override the 

 sandstone, as shown in figure 7. That the copper deposition was largely prior 

 to this folding is shown by numerous facts. A copper-bearing lode is cut off 

 by the underthrust Eastern sandstone. The Algoruah mine, the Lake, and 

 South Lake may be conceived of as working on one bedded lode folded so as 

 to be exposed three times. 



Later History 



But all this time erosion was going on, and theoretically near this fault 

 there must have been land deposits of Permo-Triassic age, some remnant of 

 which may some day be found like the fragments of Cretaceous found in the 

 Mesabi Range. 



During Cretaceous time the peneplain must have been somewhat higher 

 than the present surface. And sometime in the Tertiary, the down-cutting 



Figure 8. — Ideal Sketch of a Cross-section through the southern Part of the Keweenaw 

 (Copper) Range (ck), Limestone Mountain (j), Silver Mountain (i) to the Huron 

 Mountains. 



streams reaching harder rock through the mantles of paleozoics, became more 

 and more adjusted to the structure, though here and there transverse streams 

 persisted in valleys now occupied by Portage Lake, the Flint Steel, Fire Steel, 

 and Ontonagon. This Tertiary uplift presumably, culminated in the Quater- 

 nary when this region was overridden by the Keewatin ice, and later was 

 about the limit of the westward spread of the Labrador ice. At first the bed- 

 rock conditions are somewhat as in figure S. Near the fault-line we find both 

 thrust and vertical faults as well as slump faults, the strata disturbed and 

 broken. The thrust-faults have a flatter hade than the strata and the thrust- 

 faults a steeper. Both tend to cut out certain parts of the section. 



The present surface level' relative to the fault systems is sometimes higher, 

 sometimes lower, so that in places we find the Keweenaw formation overlying 

 the Eastern sandstone, as at k of figure S, and again upturned against it. as 

 would be the case did the present surface pass just above m of figure 8. 



Presented in abstract extemporaneous!}'. 



