346 W. H. C. SMITH — ARCHEAN rocks west of lake superior. 



Superior exhibit no trace of this or of any Lateral pressure, the inference 

 is stronu that the Steep Rock series is older than the Animikie. 



Its Unconformability not- always Observable. — The unconformity between 

 theSteep Rock series and the Lanrentian dioritic granites and anticlastic 

 chloritic schists of the north and east shore of Steep Rock lake admits of 

 scarcely any doubt, but the unconformity above the Keewatin schists of 

 the Seine river to the southwest is not at all obvious. Lithologically the 

 green altered traps and schists of the two series are strikingly similar 

 and could not probably he separated by the most careful study, hut it is 

 significant that west of the faults of Steep Rock lake the most careful 

 search on the part of both Mr Smyth and myself lias been rewarded, as 

 far as I know, by the discovery of only one exposure (and that of a few 

 square yards in extent) of rock that can with any degree of certainty he 

 regarded as representing the characteristic limestone formation of the 

 Steep Rock series. This was found by the writer on the north side of 

 Seine river, about four miles west of the mouth of the Atic Oban river. 

 Farther down the Seine river some thin bands, which are very doubt- 

 fully representatives of the upper calcareous formation (VI), are seen in 

 two or three localities, but these are of trifling extent. West of Steep 

 Rock lake and north of Seine river are some important developments of 

 traps, light greenish-gray and fine-grained, with some dynamic schists, 

 probably derived from them, which are microscopically quite similar to 

 those grouped under horizon VII. We would expect, however, to find the 

 remains of the eruptive members of this series among the lower rocks of 

 the neighborhood. It seems, therefore, that to the west the Steep Rock 

 series has been faulted up and swept away, and whatever significance 

 may attach to discordance of present structural relations on opposite 

 sides of a fault plane, these relations point to the conclusion that this 

 series lay unconformably on the Keewatin. 



The Unconformity *> }[r<i*irr< of geologic Time. — If the above interpreta- 

 tion of the structure and relations of this series is correct, it is most in- 

 teresting, as it is the most important if not the first recognized undoubted 

 unconformity in the Huronian system of the Canadian geologists. As 

 the period of time predicted by this series is enormous, it strongly 

 emphasizes Lawson's statement thai the erosion interval between the 

 Keewatin and the Animikie is the greatest in American geology. 



Its Relation to the Atic Oban Series. — To the south the Steep Pock series 

 appear to have been removed by the same causes. South of the south- 

 west arm of Steep Pock lake it appears to be cut off by the quartz por- 

 phyries of Smyth's tentative Atic Oban series, but the writer's sub- 

 sequent examinations have convinced him that the sonthwestward 

 extension of these quartz porphyries is not great and almost certainly 



