310 THE JOURNAL OF GEOLOGY. 



Keweenawan by Irving, and the Bohemian Mountains of Keweenaw Point. 

 It is suggested that the anorthosites of Lawson are but facies of the gabbro, 

 and that the two belong together in the Norian. 



Conmients : — This paper correlates with the so-called Norian of the East 

 the gabbros and similar rocks of the Lake Superior region, which have here- 

 tofore been considered as constituting a part of the Keweenawan. Such 

 lithological correlations are believed to retard, rather than advance geological 

 progress, as they rest wholly upon unverified assumptions. The-local name 

 Keweenawan ought to be retained for the gabbros and allied rocks, or else 

 some new local name ought to be devised for it. This latter is done by 

 Lawson as appears from the pap^r next summarized. 



Lawson ^ gives a petrographical and structural account of the anorthosites 

 of the Northwest shore of Lake Superior. The anorthosite is wholly massive, 

 completely granitic in structure, and is composed almost wholly of basic 

 feldspar, varying in composition from labradorite to anorthite. The rock 

 occurs near Encampment Island, in the vicinity of Split Rock point, at 

 Beaver Bay and vicinity, at Baptism river, on the slopes of Saw Teeth 

 mountain, and at Carlton Peak. In nearly all of these localities the rock is 

 found in rounded dome shaped masses below the other eruptives of the coast. 

 It is cut by these different eruptives, and in the lava flows are found very 

 numerous blocks and bowlders of anorthosite, which were caught in at the 

 time of their extrusion. These facts show that the anorthosite is of pre- 

 Keweenian age, and since the anorthosite is a plutonic rock, it must have 

 suffered profound erosion prior to the extravasation of the Keweenian 

 eruptives. Norwood, Irving and Winchell have described the blocks of 

 anorthosite in the lavas at some of the points. Winchell regarded the 

 anorthosite at Split Rock as older than the eruptives containing masses of 

 them, and Irving reached the same conclusion in reference to the anorthosite 

 at Carlton Peak. However, none of them differentiated the anorthosite mass 

 from the general aggregation of volcanic flows, constituting the Keweenian 

 series of the Minnesota coast. The surface of the pre -Keweenian anortho- 

 site is domed and hummocky like that of the other Archean terrains of 

 Canada, and it is thought to have been onl}^ modified by Pleistocene erosion. 

 The interval between the anorthosite and the Keweenian is probably the same 

 as the pre - Paleozoic interval which effected the reduction of the Archean 

 to the great hummocky plain, to which it was reduced before the Animikie 

 was deposited upon it. As the Keweenian rests directly upon the anorthosite, 

 the Animikie is absent for the middle third of the Minnesota coast. Irving 

 places the thickness of the Keweenian of the area at 20,000 feet, stating that 

 it may reach 22,000 or 24,000 feet. The maximum thickness of the 



^ Anorthosites of the Minnesota Shore of Lake Superior, by A. C. Lawson. In 

 Bulletin 8, Geol. and Nat. Hist. Siir., Minn., 1893, PP- 1-23. 



