7 THE LAKE SUPERIOR SYNCLINAL. 411 
1873, while at the same time modifying them in some respects, and develop- 
ing a number of new facts with regard to the structure and course of the 
synclinal. 
The most important modification was that with regard to the supposed 
occurrence of horizontal Potsdam or unconformably overlying sandstone 
in the trough of the synclinal. On my map of 1874 I had marked such 
a sandstone as occurring along the upper Saint Croix, as indicated by the 
descriptions of Dr. D. D. Owen,’ but this sandstone was subsequently 
shown by Sweet? to belong in the Upper Division of the Keweenaw Series, 
it being in fact but the westward continuation of the south-dipping 
sandstones of White and Bad rivers. The upper Saint Croix was again 
further examined by Sweet and Strong in 1876, and the sandstone in 
question found to be underlain conformably by fine-grained diabases 
and melaphyrs with interbedded conglomerate and sandstone. My map 
and section of 1874 had also shown horizontal sandstones filling the 
trough of the synclinal in the Bad River country. This conclusion was 
based on an observation by Dr. I. A. Lapham, which subsequent ex- 
amination by myself failed to verify. There then remained, to indicate 
the presence of this newer sandstone in the trough of the synclinal, only an 
exposure of flat sandstone on the shore of Lake Superior at Clinton Point, 
four miles west of the mouth of Montreal River; and this, as shown on a 
previous page, is rather to be regarded as the eastern termination of the 
horizontal sandstone of the Apostle Islands and of the coast of Bayfield 
County. 
The chief new developments as to the structure and course of the syn- 
clinal, resulting from the later work of the Wisconsin Survey, were (1) the 
connection by Strong of the Keweenaw Range of north-dipping rocks with 
the similar rocks of the Saint Croix by exposures all across the previously 
wholly unexamined interval between that river and Numakagon Lake; 
(2) the determination of the comparative flatness of the northward dip 
across this interval; (3) the determination by Chamberlin of a curve to the 
southward of the belts of this range, with a flat westerly dip, in the imme- 
1Geological Survey of Wisconsin, Iowa, and Minnesota, p. 161. 
2“‘Notes on the Geology of Northern Wisconsin,” by E. T. Sweet. Trans. Wis. Acad. Science, 
Vol. III, 1876. 
