THE BASIC MASSIVE ROCKS, ETC. 689 



of the difficulties now surrounding them will disappear. At 

 present the main results reached by the field-geologists who have 

 busied themselves with the rocks under discussion will be 

 referred to. They must pass unchallenged except in the few 

 cases where the microscopic evidence is directly at variance 

 with them ; and when there is no field evidence directly substan- 

 tiating them. At some time in the near future it is hoped that 

 an opportunity will offer itself for a more detailed study of the 

 rocks in the field. Then it will be proper to criticise the conclu- 

 sions arrived at by previous workers, and to suggest new views as 

 to the position and relation of the eruptives with respect to the 

 rocks with which they are associated. 



B. The Position of the Gabbro. 



The great gabbro mass which is the subject of this paper has 

 been placed by Irving in the Keweenawan group, the separation 

 of which from the underlying Huronian slates and quartzites and 

 the overlying Cambrian sandstone, is due principally to the 

 investigations of Brooks, Pumpelly, Irving and Chamberlin. 

 The history of the discussion which has led to the recognition of 

 the great Keweenawan series it will not be necessary to outline, 

 as it is well given in the essays, whose authors have been named. 1 



The only detailed description of the series as a whole has 

 been given us by Irving, 2 who makes it "include only the suc- 



1 It should be stated here that although the individuality of the copper-bearing 

 series of rocks is recognized by nearly all geologists who have worked in the Lake 

 Superior region, several have declined to regard it as a distinct series, equivalent to 

 the Huronian or the Cambrian. These geologists prefer to look upon it as belonging 

 with the latter group as its lower member. Dr. Wadsworth has long held this view, 

 and Prof. N. H. Winchell (8th Ann. Rept. Geol. and Nat. Hist. Survey of Minn., p. 22 ; 

 17th ibid., pp. 54-55) in one of his most recent reports sums up the work of the Min- 

 nesota Survey in this direction in the statement that the Keweenawan series is closely 

 linked with " the great gabbro flow," to which reference will be made hereafter, and 

 that both are members of the Potsdam. In a later report (20th Ann. Rept. Geol. and 

 Nat. Hist. Survey of Minn., p. 3) the same writer discusses the age of the gabbro and 

 concludes that it is much older than the Potsdam, but he does not assert positively that 

 the Keeweenawan beds overlying it are pre-Cambrian. t 



2 The Copper-Bearing Rocks of Lake Superior, R. D. Irving : Monograph V., U. 

 S. Geol. Survey, Washington, 1883. 



