448 THE JOURNAL OF GEOLOGY. 



within them in isolated pieces. In a similar manner small areas of Lauren- 

 tian granite, sometimes directly in contact with the schists, have the imper- 

 fectly crystalline condition of the Keewatin. 



Nos. 3 and 4 are separable from No. 2 by divergence in dip and strike, 

 as well as by a marked difference of lithology. There is consequently some 

 evidence of unconformity between them. Below No. 4 is a great physical 

 break, which separates Nos. 1,2, 3, and 4 from 5, 6 and 7 throughout the Lake 

 Superior region. This break is the greatest erosion interval which has been 

 discovered in Palaeozoic geology, i, 2, 3, and 4 together constitute the 

 Taconic, Nos. 5, 6, and 7 constitute the fundamental complex or Archean, 

 which is a unit in its grander features. 



The structure and origin of the foregoing series are considered in some 

 detail. It is concluded that stratification can always be discriminated from 

 schistosity or slaty cleavage by the varying shades of color bands which sweep 

 across the surface of the rocks, and by gradations in the kind and size of 

 grains across the bands. These layers may vary from 1-16 of an inch to 

 several inches or several feet across. 



Comments. — As used by the United States Geologists, Nos. i, 2 and 3, are 

 included in the Keweenawan. These divisions and the break between 2 and 

 3 are recognized by Irving, so that the difference is merely one of nomencla- 

 ture. No. 4 is Upper Huronian ; No. 5 is Lower Huronian ; and Nos. 6 and 

 7 are the Basement Complex or Archean. The break between the Lower 

 Huronian and the Basement Complex is perfectly clear on the south shore of 

 Lake Superior, and is found by Lawson at the base of the Keewatin in 

 Ontario. In Minnesota, Professor Winchell, on the contrary, regards the 

 Keewatin as grading down into the underlying series. Many geologists would 

 disagree with the statement that stratification can always be discriminated 

 from schistosity or slaty cleavage by either of the criteria mentioned or by 

 both combined. 



Grant,^ in 1893, publishes his note book, made on a trip in Northeastern 

 Minnesota. The areas visited were those of the Kawishiwi river, Snow 

 Bank lake, Kekequabic lake, and Saganaga lake. In the study of these 

 areas there was no evidence found of a transition from semi-crystalline 

 and crystalline schists into granite. On the other hand, abundant evidence 

 was found of the eruptive nature of the granite rocks into the surrounding 

 sediments. The gneissic and so-called bedded structure in the granitic rocks 

 is not as common as has been supposed, the structure usually being truly 

 granitic. The Kawishiwi river and Snow Bank lake massive rocks are horn- 

 blende syenites. The Saganaga rock is a coarse hornblende granite. That 



' Field Observations on Certain Granitic Areas in Northeastern Minnesota, by U. S. 

 Grant. In 20th Annual Rep. Geol. and Nat. Hist. Sur. Minn., pp. 35-110. 



