GENERAL GEOLOGY OF THE DISTRICT. 



.463 



rock to be in a horizontal position. The Penokee series is not al^sohitely 

 cut off" by the sandstone, but its breadtli at one point in the northwest part 

 of Sec. 28, can not be more than 200 or 300 feet, and it is probable that 

 farther south the entire succession is overlain by it. 



It is clear, then, in the upward passage to this Eastern sandstone that 

 another great unconformity has been passed. First, the sandstone is in a 

 horizontal position, lying directly upon the upturned edges of the Penokee 

 series and presumably having the same relations to the Keweenaw series. 

 Second, this sandstone contains abundant fragments derived from the 

 Southern Complex, Penokee-Gogebic, and KeweenaAv series. From what 

 has gone before it is manifest that in order that this should be the case the 

 whole Keweenaw and Penokee series must have been tilted and have suffered 

 vast erosion in order that the base of the latter and the underlying series — 

 once probabh' buried under many thousands of feet of sediments — could 

 reach the surface and furnish these fragments. Since the sandstone is hori- 

 zontal it is plain that the two upper series were tilted to their present 

 inclination and eroded before the deposition of this sandstone. The final 

 part of the erosion furnished the material of which the sandstoiu^ is niade. 

 This unconformity is one, then, which in magnitude is probabh- equal to 

 if not greater than the geological break between the Penokee series and 

 the Southern Complex.^ 



Resume of f/coloffical histonj. — We are now prepared to give a resum^ 

 of the geological history of the district of which the Penokee series is 

 a part. The oldest rocks are the great Southern Complex. ( )f the origin 

 of a part of them little is known, but, whatever their genesis, before 

 the beginning of the deposition of the Penokee series there was a long 

 period during which earth movements and erosion acted upon tlieiii. This 



' The discordance hctivcen tlii^ eastern sandstniie and the Keweenaw series lias lieiMi Jdu;; niaiu- 

 taini'd !>% eminent genhieigts, th(inj;li denied by (ithers. Tliis imsitiiin was first talien liy IJrooks and 

 rnnipelly in 1S72 (On tlie Aj^e of the Copper- Bcarini; Koeks of Lake Superior. .\ni. .lonr. Sci., 3d 

 series, vol. iii, pp. 128-432). The last exhaustive an<l eonviuiing treatiuent ol' this (|nestiou is hy 

 Profs. R. D. Irving and T. ('. Chanilierliii (Bnll. l'. S. Geol. Survey Xo. 23). Ujion Keweenaw point it 

 has been maintained that the pebbles in the samlstone do unt ini])ly a great lireak lieeaiise of <Muptive 

 origin. At Oogebic lake are nearly all thi' phases ol jiebbles of the three great series of rock of the 

 region. That they shoulil be alinudanlly contained in Ihe sandstone in tlie condition which they are 

 now found i» .«|7« can not be explained upon any other Iiypothesis than a gnat unconformity. 



