AUGUST 21, 1896.] 



SCIENCE. 



219 



Lake, but the majoritj^ of the smaller streams 

 flows to the north and to the south from the 

 central trap area, and they are thus conse- 

 quent. 



The west branch of the Ontonagon river 

 flows from Agogebic lake along the south- 

 east border of the trap range for 20 miles 

 or more. Here it joins the other main 

 branch and the river breaks directly across 

 the trap range in a southwesterly direction. 



The only transverse cut where the trap 

 range is reduced almost to the level of Lake 

 Superior is at Portage Lake, and this place 

 has been utilized for a ship canal. From 

 Portage Lake the banks rise steeply from 

 500 to 700 feet, nearly to the baselevel 

 above. No explanation for this exceptional 

 reduction has been offered. One is tempted 

 to believe that here must have been unusual 

 fracturing or faulting, and this idea is en- 

 couraged by the presence adjacent to Por- 

 tage Lake of a number of important copper 

 mines on the amygdaloids. It is well 

 known that the amygdaloid mines occur 

 where there has been much crushing of the 

 porous rocks, as a result of the differential 

 movement between the layers of trap. A 

 partial explanation of the Portage Lake 

 gap may be the comparative narrowness of 

 the range at this point, as a result of steeper 

 dips. This increased steepness of dip im- 

 plies greater accommodation between the 

 layers, and therefore more fracturing of the 

 rocks. 



The Little Montreal river, which rises on 

 the trap range, flows in a nearly east and 

 west course for 15 miles in one of the 

 softer divisions of the Keweenawan series 

 between two harder divisions, before 

 turning abruptly to the south and 

 breaking through the porphyries, felsites, 

 and traps. Had it continued four miles 

 farther, in a course little south of east, it 

 might have reached Lake Superior at the 

 end of Keweenaw Point without breaking 

 through the resistant formations. A close 



examination of Irving's Plate XVII. of 

 Monograph Y . referred to shows many other 

 interesting points in reference to the drain- 

 age. 



In a recent number of Science, July 17, 

 1896, I described a central Wisconsin base- 

 leveled area, more nearly perfect than that 

 at Keweenaw Point. From center to center 

 of the two districts is about 150 miles. The 

 central Wisconsin district has not been so 

 deeply dissected as Keweenaw Point, but 

 this is readily explained by the fact that it 

 is not so near either of the Great Lakes, and 

 therefore erosion has not so thoroughly 

 stretched its fingers over it. The central 

 Wisconsin baseleveled plain is at an alti- 

 tude of about 1450 feet. The Keweenawan 

 baseleveled plain is at an altitude of about 

 1350 feet. Therefore the baseleveled areas 

 of the two districts are probably but parts of 

 a far more extensive baseleveled region. 



The area intervening between the two 

 districts consists of Huronian and Archean 

 rocks. Resistant quartzites and mica- 

 schists are characteristic rocks of the 

 Huronian, and gneissoid granite is the 

 dominant rock of the Archean. Since the 

 most resistant rocks were not reduced to 

 the sea level at Keweenaw Point or in cen- 

 tral Wisconsin, one would expect that the 

 more widespread, equally resistant rocks of 

 the Huronian and Archean would also pro- 

 ject above the baseleveled plain . As a matter 

 of fact topographic maps of the Marquette 

 and Penokee districts made by the U. S. 

 Geological Survey, and under my own di- 

 rection of the Michigamme district south- 

 west of the Marquette district, show that 

 extensive tracts of country are at altitudes 

 from 1600 to 1800 feet or more, thus verify- 

 ing the expectation. The variations of 

 level of the ancient uneven plain through- 

 out the region, however, are probably not 

 so great but that Davis's term peneplain 

 may not properly be applied to it. So far 

 as I know, H. L. Smj^th was the first to call 



