APPENDIX. 



(A) 

 THE STRUCTURE OF THE KEWEENAWAN. 



A large mass of notes has accumulated in the course of the field work, as well 

 as in the subsequent study of the rocks of the Keweenawau, bearing on the succes- 

 sion of the parts of that formation. It was once designed to review that subject 

 more thoroughly than has been done in the discussion in Part I. There are sundry 

 references in the body of Part II to such intended review, but the chief points only 

 will be presented, with short references to preceding pages where illustrative facts 

 are mentioned. 



1. The tendency of the evidence has been to increase the bulk of the surface 

 lavas that preceded the Beaver Bay diabase, and hence toward the separation of the 

 Beaver Bay diabase from the gabbro mass of the northern part of the Keweenawan 

 belt in Lake and Cook counties, in chronologic birth as it is in geographic place. 

 (Compare Nos. 522, 523 and 525.) It seems, therefore, that there are two main series 

 of alternating traps, sandstones and amygdaloids, one in the Cabotian and one in the 

 Manitou. The knowledge we possess of the geology of the country immediately 

 back from the shore line of lake Superior is too meagre to warrant an attempt to 

 define these two series. 



The Beaver Bay diabase may be credited, therefore, with some of the phenomena 

 which, along the lake shore, have been ascribed to the earlier gabbro, such as the 

 production of red rock, as at Grand Portage, and perhaps at Pigeon point; the pro- 

 duction of the great dikes at Grand Portage and the great sills of the international 

 boundary in Cook county. This great mass has very widely been called a part of 

 the original gabbro, and it cannot at present be separated from it either geograph- 

 ically or petrographically, although it seems to be separate from it stratigraphically. 

 They both belong in the Cabotian, as defined, but the Beaver Bay diabase may have 

 to be separated from the Cabotian. 



2. The age of the Logan sills. The age of the sill north of Birch island (Nos. 

 268, 265) is the same as that of the great east- west dikes. The latter are earlier than 

 the conglomerate at the base of Grand Portage island, as their debris and the "red 

 rock " which they formed are in that conglomerate. Therefore, the sills of the region, 

 and probably all the Logan sills, are not later than the Keweenawan. 



