1884.] 173 [Wadsworth. 



the sea — an unconformability which indicates sequence of time 

 but not difference in geological age. 



The one vexed question over which geologists have been exer- 

 cised is : are these copper-bearing rocks pre-Potsdam or not? On 

 every hand it has been admitted that the sandstone to the east- 

 ward of those rocks on Keweenaw Point is of Potsdam or more 

 recent age — the weight of authority being at present in favor of 

 the Potsdam. Again the relation of that sandstone to the inter- 

 calated lava-flows, sandstones, and conglomerates is looked upon as 

 decisive proof of the age of the lava. Two strong parties of geol- 

 ogists have discussed this question — one claiming that the east- 

 ern sandstones were of the same age as the lava flows, the other 

 that the latter formed a distinct and older series. 



The principal evidence in support of this latter opinion was ob- 

 tained at the Douglass Houghton Falls near Torch Lake. Here it 

 was stated that the horizontal eastern sandstone extended up to 

 the base of the falls, the rock forming the cliff over which the 

 river runs being the old basaltic lava. It was further declared 

 that the cliff at the falls formed an old seashore bluff and that 

 the sandstone and conglomerate abuting against this contained 

 its water-worn debris. Here on its face was good evidence that 

 the traps were older than the eastern sandstone and of greater 

 geological age. This was the only evidence of importance that 

 originally caused the copper rocks to be named Keweenawan and 

 regarded as pre-Potsdam. 



In 1879 the present writer examined this locality, studying not 

 only the bed of the stream but also the steep banks of the ravine 

 below the falls. He found that the accounts of the structure here, 

 that every geologist who had examined the district for thirty 

 years previously had given, were according to his observations in- 

 correct. The sandstone was not horizontal or at most clipping 

 from one to five degrees, but the dip gradually increased up to 

 25°, until the falls were reached. Furthermore these northwesterly 

 dipping sandstones and conglomerates, that every one until that 

 day had conceded to be truly the eastern sandstones were found to 

 contain intercalated lava flows and to possess the structure of the 

 typical copper-bearing rocks. These observations fully explained 

 the occurrence here of the basaltic debris, and showed that it came 

 from the underlying lava flows instead of from the hypothetical 



