GENERAL GEOLOGY OF THE DISTRICT. 461 



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1,000 feet less. Probably the series must have been eveiywhere thicker 

 than at the present time at any place. The amount of erosion during this 

 period was, then, in some places measured perhaps by twice 10,000 feet. 

 During the progress of this erosion it is possible that the series was gently 

 bent into a synclinal trough, the center of which was in the . vicinity of 

 Tylers fork and the eastern end near Sunday lake. This suggestion is 

 made as an explanation of the difference in the amount of erosion, 

 those parts naturally being eroded most which were at highest elevations. 

 The time gap, then, between the Penokee and the Keweenaw series 

 must have been sufficient for a widespread orographic movement and deep 

 denudation — a vast lapse of time ; so that while the unconformity between 

 these two serie.s is not nearly so great as between the Penokee series and 

 the Southern Complex, yet it stands as one of the greater time gaps in 

 " geological histoiy. 



The Eastern sandstone, and the unconformity at its 6asf'.— The only remain- 

 ing terrane in wliich we are concerned in this memoir is tlie Eastern sand- 

 stone, which (as sliown by PI. i), extending from Keweenaw lja\- to a long 

 distance west of Gogebic lake, conceals a broad .strip of the older forma- 

 tions. The low ground north of the trap range, in the north part of T. 47 

 N., R. 44 W. and 43 W., Michigan, is the soutliern boundarv of this sand- 

 stone. As T. 47 N., R. 42 W., Michigan, is reached, near Gogebic lake, 

 this low ground swings rapidly to the southward, and it was early in this 

 studv suspected that hei'e the Eastern sandstone is the surface formation, 

 but no natural exposure was found which would either prove or disprove 

 this suspicion. However, test pitting in the NW. ^ of Sec. 28, T. 47 N., R. 

 42 W., Michigan, lias shown tlie sandstone to there exist. The l)asal con- 

 glomerates there found at the junction of the Penokee series with t\\6 

 underlying granites have already been described, }ip. 304-3!)"), 409. In 

 this interesting locality is still another basal conglomerate which belongs to 

 the Eastern sandstone. In the test pits close to the outcrops of the Peno- 

 kee series and the Southern r*om])lex the conglomerate is coarse, and its 

 matnx is somewhat indurattMJ, it Iteing necessary to resort to blasting in 

 sinking. U})on exposure to the nir the matrix of tlie conglomerate dis- 

 integrates, as a result of wliich a lieaj) of sand, pebbles, and l)o\vlders of 



