hi'l -\V. O. HOTCHKISS THE LAKE SUPERIOR GEOSYXCLEsE 



long-drawn-oiit •'pipe*' amygdiiles nearly perpendicular to the base of 

 the flow. These were believed to be due to steam produced by water 

 present in the conglomerate. The tops of these pipe amygdules were 

 uniformly bent up the dip. indicating that just before solidifying the 

 lava was moving upward in relation to the present dip. or. in other words, 

 that the slope on which it flowed was down to the south and east. One 

 case of cross-bedded sand in the ba^e of the conglomerate was called to 

 my attention by Doctor Lane. This showed that the depositing water 

 current at that point was flowing in the same general direction — down- 

 ward to the southeast. 



Though it may not be held that the evidence is conclusive that the 

 original Keweenawan slopes in the Gogebic district were all downward 

 to the south and east, nevertheless Avhat evidence is available is practi- 

 cally all in agreement with this conclusion and justifies as a working 

 h}*pothesis the assumption that the source of the flows was in the present 

 bottom of the syncline. Further evidence is needed, and. whatever its 

 character, will doubtless be forthcoming in future geologic work on these 

 formations. 



EVIDZXCES OF TlLTIXG OF THE LaXD SURFACES 



A cross-section from the Archean baselevel to the Upper Keweenawan 

 sandstones through Bessemer was made by Gordon for the Michigan 

 Geological Survey and published in 1906. This is ty*pical for the whole 

 range and shows the Iron wood dipping north 60 degrees, the lowest 

 Keweenawan flow dipping north about 80 degrees, and a gradual lower- 

 ing of the dip to 30 degrees in the outermost flows. 



If we accept as a working hypothesis that the flows were poured out 

 on a south-sloping surface from a source to the north of the present out- 

 crop, then the iron formation must have been tilted at the time of the 

 first flow so that it had a southward dip of 20 to 30 degrees. This indi- 

 cates a rising of the land to the north, which had apparently been 

 progressing all through the deposition of the Tyler, as we find the north- 

 ern outcrops of that formation often dipping more steeply than the iron 

 formation and the ba-sal beds of the Tyler. 



If the source of the flows continued to be the same — north of the pres- 

 ent outcrop — then the greater elevation of the surface to the north must 

 have been maintained continuously throughout the accumulation of the 

 flow series. The surface of each succeeding flow must then have marked 

 a gentle southward slope at the time of its outpouring. This being the 

 case, the decrease in present dip of the upper flows as compared to the 

 lower must indicate a Sfradual tiltinsr of the lower flows and the Huronian 



