Page:The Journal of geology (IA journalofgeology21894univers).pdf/464

7. within them in isolated pieces. In a similar manner small areas of Laurentian granite, sometimes directly in contact with the schists, have the imperfectly 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 Palæozoic geology. 1, 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. 1, 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 nomenclature. 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,^[1] 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 hornblende syenites. The Saganaga rock is a coarse hornblende granite. That