PROGRESSIVE TRANSITION OF ROCKS. 385 



scales appearing like sericite, but in places recognizable as mica. Here, 

 then, are alternating sheets of porphyrel and crude uralitic and micaceous 

 schist. Now appear very thin laminae composed of feldspar, a hornblende- 

 like mineral, and ten per cent, of quartz. These continue to alternate with 

 the porphyrel. The alternations become exceedingly frequent, but the ura- 

 litic bands increase in breadth, and the whole terrane finally becomes a 

 uralitic gneiss, and soon after reveals the "coarse quartz individuals of the 

 well-known Saganaga gneiss. 



It is hardly necessary to remark that the transitions mentioned are simply 

 progressive in a geographical sense. The historical or genetic succession 

 may have been the reverse, or the whole work may have been simultaneous. 



Thus far the older rocks of the Northwest have presented a condition of 

 strict structural conformity. This fact was long unsuspected by American 

 geologists, but no field geologist of the Northwest entertains on this subject 

 the least doubt. What is even more striking is the gradual transition and 

 mutual blending witnessed in the passage from one of the systems enumer- 

 ated to the contiguous one. The facts provoke many theoretical inquiries ; 

 but I will only state that I do not regard the universal conformity of strati- 

 fication as evidence of the absence of geological breaks. 



The Uncrystalline Schists. 



On the north side of Gunflint lake the vertical schists are found overlain 

 by schists extremely different in character and attitude. They are nearly 

 horizontal, having a dip here of only about five degrees south by east. 

 They undergo a great development in northeastern Minnesota. They have 

 been traced westward well toward Ogishke-muucie lake. Eastward I have 

 traced them to Partridge falls ou Pigeon river. Between the national 

 boundary and Thunder bay they have been reported by Dr. Robert Bell, 

 of the Canadian Survey,* and on Thunder bay and in its vicinity they 

 have been described repeatedly by the Canadian observers. They constitute 

 the "Animike series " of Hunt, and by that name I shall for the present 

 refer to them. In Minnesota this is strikingly a thin-bedded, black argil- 

 litic series, rising in high bluffs along the south sides of the lakes and gen- 

 erally crowned by twenty-five to seventy-five feet of semi-columnar gabbro. 

 These characters present themselves at all outcrops as far as Thunder bay. 

 Certain strata, not very definite in position, receive dissemiuated grains of 

 quartz, and the formation thus approaches a proper black graywacke. 

 Within this system other strata, more definite in position, acquire a siliceous 

 character, and some become strictly beds of flint and jasper schist. Some of 

 these are brilliantly red or deep black, smoky, yellow, or chalcedonic. The 



* Report for 18G6-'9, p. 322. 



