No. 2.] SPENCER — COPPER-BEARING ROCKS. 67 



but a gigintie fault, then we should expect to fiid the deposits 

 of the Nipigon Period, between the Huronian and subsequent 

 sedimentary rocks father east along the south shore of Like 

 Superior, but this is not the case. The trend of the rocks on 

 Micliipicoten Isl .nd is very nearly the s une as that on Keweenaw 

 Point, although they dip in the opposite direction. From this 

 difference in direction of dip, and from the d/.ep w iter between 

 the Point and the Island, I wou'd infer th it the old Cupriferous 

 Range was much weikened and broken between these two pi ices, 

 and subsequently was easily swept away by denuding agencies, 

 leaving this deep portion of the 1 .ke b isin. 



The south-ea-tern portion of the Lake Superior Basin does not 

 appear to h <ve been covered with the depo-its of the Nipigon 

 or Cup. iferous Series but to h ive been exc .vat d along the junc- 

 tion between the Nipigon Hills to the n n-tlnvard (which have 

 nearly been swept away) and the softer sedimentary rocks to the 

 south. 



From the foregoing we see that part of the Huronian Series 

 had not been upturned before the eruptions of the Nipigon or 

 Cupriferous Age became general. It is probable that the dyna- 

 mic .1 agencies, which were exhausting t'aem-e.ves by covering up 

 the Huronian sea bottoms to the south with igneous ui itter, were 

 also at work upheaving the oider rocks to the north — inferring 

 that the eruptions began somewhat earlier to the southward of 

 the series. This view is strengthened by the face that the earlier 

 beds of this foruntioii on the North Shore are mo-tly made up 

 of sedimentary deposits derived from the waste of the older cry- 

 stalline rocks, with only an occasional trap overflow — the record 

 of some extraordinary eruption. 



The Hurouiau and Cupriferous Series were elevated together, 

 having the greatest upheavals to the south, where the beds were 

 lifted to angles of from 50 to 70 deg., while toward the north tli3 

 inclination is comparatively low. Now this fact, with the absence 

 of the great capping traps to the southward, together with their 

 unconfnrm ability to the older beds of the Copper- Bearing Series 

 to the north of Lake Superior, would tend to show th it the seat 

 of volcanic eruptions moved northward, and ended there later 

 than to the south, while the upheaving forces were acting in the 

 opposite direction. The last upheaval to which the south shore 

 of Lake Superior was subjected, probably occurred shortly pre- 

 vious to the deposition of the Potsdam sandstones. 



