418 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
as present knowledge will allow, it has been discussed in the previous chap- 
ter. 
In the preceding generalized hypothetical section of the Lake Superior 
basin, which may be looked on as taken across from the Pigeon River region 
of the North Shore, through Ontonagon, the South Range, and the Meno- 
minee region of Michigan and Wisconsin, but not on a straight line, and 
not drawn to any scale, I have attempted to bring out the following points: 
(1) the synclinal structure of the lake basin; (2) the partial unconformity 
of the Keweenawan to the unfolded Huronian; (3) the supposed relations 
of the folded and unfolded Huronian; (4) the limitation of the Keweenawan 
outwards by the higher Huronian land; and (5) the origin of the Keweenawan 
eruptive rocks through fissures arranged around the rim of the trough. If 
this sketch represents actual conditions, then the downward bowing of the 
great trough, which subsequently was filled with the Keweenawan accumu- 
lations, was begun in the Huronian and continued through the Keweenawan. 
Accompanying this downward bowing was a crumpling of the Huronian to 
either side of the broader bow—and this crumpling, so far as this sketch is 
concerned, may have taken place in large measure before the Keweenawan 
—and an extravasation of molten matter around the rim of the trough. 
