464 THE PENOKEE IRON BEARING SERIES. 



erosion coutiiiueil until tliey were reduced nearly to a i)lain throug'hout 

 the distance from Numakagon lake to lake Gogebic. The district was 

 then submerged. The conditions prevailing were quiet ones, for almost 

 immediately there began forming at the bottom of the sea the nonfrag- 

 mental rocks of the Cherty limestone member. 



Between the Cherty limestone and Quartz-slate there was an ei'osion 

 interval which may mark a time break of considerable magnitude. It is 

 even probable that above the Cherty limestone member were deposited 

 other formations which have been entirely removed. While in the lower 

 part of the Quartz-slate member a considerable amount of material — even 

 sufficient to form basal conglomerates — has been derived fromthe Cherty 

 limestone, the great mass of the material came from the gneiss-granite 

 Basement Complex. Where the Chei'ty limestone is now absent the basal 

 conglomerates found contain fragments which are almost wholly from the 

 granite and gneiss. These conglomerates have been discovered at a num- 

 ber of localities and may be nearly continuous. The character of the 

 sediments was very uniform for some time, although there were to a certain 

 extent variations in conditions, thin beds of feldspathic sandstone being 

 interlaniinated with beds of shale. 



After a time thei'e was a clearing u[) of jhe waters and an assorting of 

 materials, as a result of which well rounded quartz grains only were 

 deposited, and this layer now constitutes the pure vitreous qiiartzite com- 

 posing the upper 50 feet of the member. 



After the deposition of this thin layer of sandstone there was again a 

 change of conditions by which fragmental sedimentation ceased and the 

 nt)nfrag'mental chonical or organic sediments of the iron belt beii'an to 

 form. During this period the conditions were again uniform, the material 

 everywhere, except in the Eastern area, at all horizons appearing to he a 

 cherty ferriferous carbonate. During the time of, the accunndation of the 

 800 feet of the Iron-bearing member it is probable that the bed of the 

 ocean continued steadily to subside. 



Again ii changt^ of conditions came about, resulting in the deposition 

 of fragmental layers. The passage from the nonclastic iron formation to 

 the clastic slates was not so a)>rupt as the change from the (juartz-slates to 



