218 



SCIENCE. 



[N. S. Vol. IV. No. 86. 



miles, and to the northeast Isle Eoyale 

 may been seen. Again ignoring the imme- 

 diate foreground, there is an impression of 

 almost absolute horizontality, with the ex- 

 ception of the Porcupine mountain mass, 

 to the southwest, and of some peaks on 

 the south side of the main range far out 

 toward the end of Keweenaw Point to the 

 northeast. These northeastern monadnocks 

 are supposed to be parts of the Gratiot 

 Bluff, Mount Bohemia, and Mount Hough- 

 ton range, which is known to consist of 

 hard quartz-porphyry and felsite. 



The immediate foreground both at Eock- 

 land and at the Quincy mine is exceedingly 

 rough, and these two places are typical of 

 the range. When traversed the range is 

 found to be cut by steep ravines, to be 

 carved into bluffs and hills, and everywhere 

 one is ascending or descending, The ap- 

 parent plain is evidently composed of the 

 higher points of the range, which rise just 

 about to the altitude of the ancient base- 

 level. Scarcely a remnant of the plateau 

 which once must have existed is left. As 

 determined from the topographic map of 

 the outer part of Keweenaw Point, made 

 by the United States Lake Survey, the 

 Lake Superior baseleveled area is at an 

 elevation of about 1350 feet. The culmi- 

 nating points from near Eagle Harbor 

 east run as follows : 1349, 1344, 1292, 1330, 

 1312, 1335, 1330 feet. The last is East 

 Bluff, and this is only about 4 miles from 

 the end of the Point. Upon account of the 

 northwest dips these and most of the other 

 bluffs of the district have comparatively 

 gentle slopes in that direction and steep 

 slopes to the southeast where the layers are 

 broken across. Such a remarkable uniform- 

 ity as given above in the height of peaks 

 carved from tilted rocks of varying hard- 

 ness could not be the result of erosion of an 

 elevated area. The only satisfactory ex- 

 planation yet offered for such phenomena is 

 the standard one of an elevated, baseleveled 



plain which is undergoing a second cycle of 

 degradation. The culminating points of the 

 south part of the range which rise above the 

 baseleveled area are as follows : A point 

 about 3 miles south of west from Gratiot 

 Bluff, 1534 feet ; Gratiot Bluff, 1435 feet ; 

 Mount Bohemia, 1469 feet ; Mount Hough- 

 ton, 1429 feet. These points therefore rise 

 from 100 to nearly 200 feet above the base- 

 leveled area. According to Irving, many of 

 the ridges of the Porcupine mountains have 

 elevations of 1600 to 1800 feet, while but 

 small parts of the lower portions are as low 

 as 1400 feet. This places the higher points 

 of the Porcupine mountains from 250 to 

 450 feet above the baselevel. 



For the greater part of the area the pres- 

 ent cycle of erosion is evidently at its full 

 maturity, and for the outer part of the Point, 

 from which the altitudes above given are 

 taken, just past that stage. 



The Potsdam sandstone to the southeast 

 and the upper division of the Keweenawan 

 to the northwest of the main trap range, on 

 account of their softness, are everywhere cut 

 below the ancient baseleveled plain. These 

 areas, unlike the trap range, are in large 

 part so nearly reduced to the level of Lake 

 Superior as to show very much less irregu- 

 larity of surface than the main trap range. 

 Although cut by river valleys and ravines, 

 and although there are slopes everywhere 

 and in many places very considerable irregu- 

 larities of surface, the comparative flatness, 

 as contrasted with the Keweenawan rocks 

 of the main trap range or with the Huronian 

 and Archean rocks to the south, is very 

 marked. 



A cursory glance at the maps of the dis- 

 trict in question shows that when topo- 

 graphic maps of the entire area are made, and 

 its drainage studied, numerous interesting 

 features will probably appear. The trap 

 range is traversed by the stonger streams, 

 such as the Presqu' Isle, the Ontonagon, the 

 Fire Steel and Flint Steel rivers, and Portage 



