CHAPTER IV. 
PHYSICAL STRUCTURE AND GEOLOGY OF THE NORTHWESTERN AND WESTERN 
PORTIONS OF THE VALLEY OF LAKE SUPERIOR. 
SECTION I. 
PRELIMINARY REMARKS. 
In connexion with some notes on the physical structure of the country lying be- 
tween Pigeon River and the Grand Portage of St. Louis River, on the north, and 
Bois Brulé River and Fond du Lac Supérieure, on the south, and forming part of 
the western portion of the Valley of Lake Superior, it is deemed not out of place 
to call to recollection a few facts with regard to the structure of certain portions of 
the Western and Northwestern States, which tend to confirm and elucidate, in my 
opinion, the geological observations made in the region of the great Lake, and to 
support the conclusions drawn from them; and also to throw some light on several 
obscure points in the geology of certain other portions of the Chippewa Land Dis- 
trict. I shall not, however, enter into minute details, nor attempt to discuss, here, 
the conclusions which these facts have given rise to; nor will an endeavour be made 
to fortify them by the adduction of numerous facts of a similar character, which 
might be brought forward from every portion of the valleys of the great lakes, as 
well as from the entire Valley of the Mississippi. A simple reference to them will 
be sufficient to suggest to every one their applicability to the subjects of this 
chapter. 
As what I conceive to have been great valleys in the rocky strata of large por- 
tions of Wisconsin and Minnesota, have been filled up, and the country, in a great 
measure, levelled by the accumulation of immense deposits of drift, it is not pos- 
sible to determine, with anything like accuracy, the -width of the original valleys, 
nor the exact lines of the anticlinal axis separating them; but the distances from 
one synclinal line to another may be ascertained, now, with as much precision as 
the linear surveys of that region, together with the drafts of the principal streams 
in the unsurveyed portions of territory, by members of the Geological Corps, will 
