PRE-KEWEENAWAN FORMATIONS 691 



rocks are lava flows, volcanic tuffs, conglomerates, agglomerates, and slates, 

 with all sorts of gradations, just such as one would expect if a volcano rose 

 in a sea and volcanic action continued for a great period. Naturally in this 

 area it is not possible to map any continuous sedimentary belts. The dominant 

 rocks are greenstone conglomerates and lavas and massive eruptives. The 

 uppermost formation for the extreme eastern part of the area is ferruginous 

 slate. This ferruginous slate, though dominantly clastic, contains narrow 

 bands of non-clastic sediments such as chert, cherty ferro-dolomite, ferro- 

 dolomitic chert. It is believed that the ferruginous slate is probably at 

 the same horizons as the Ironwood formation to the west and that its 

 dominant fragmental character is due to the presence in this area of one or 

 more volcanic mountains which arose above the water and upon which 

 the waves were at work after the close of the period of active volcanic 

 outbreaks. 



SUMMARY OF THE WRITERS' CONCLUSIONS 



The writers' most important conclusions, in so far as they differ 

 from those of the earher writers as summarized above, may be 

 stated briefly as follows: 



1. Huronian. — There are three unconformable sedimentary groups 

 in the Huronian series. The formations heretofore included in the 

 Upper Huronian (Animikie) group are divisible into two groups 

 separated by an unconformity of the first magnitude. For the 

 superior group we propose the name Copps, in recognition of the 

 important exposures of its basal horizons near the old Copps 

 mine. ' , 



2. Archean. — -The granite heretofore described as Laurentian not 

 only intrudes the Keewatin but also cuts across two great unconformities 

 into the Animikie group. Some of the granite may be Laurentian, 

 but we have not been able to so correlate any of it. 



3. The unconformity between the Copps and Animikie groups is 

 one of great angular and erosional magnitude and is of greater impor- 

 tance in this district, measured by criteria for evaluation of breaks 

 in pre-Cambrian sedimentary successions, than either the uncon- 

 formity separating the Keweenawan and Huronian or the one 

 separating the Animikie and Lower Huronian. 



For convenience of reference and greater clearness in the dis- 

 cussions which follow we introduce the successions of the United 

 States Geological Survey and the writers' in parallel columns. 



