THE KEWEENAWAN. 417 



CONCLUSIONS AS TO AGE AND RELATION OF THE GABBRO AND SILLS. 



In conclusion, there is pi-esented the following summary of the facts 

 observed which seem to indicate the true relationship between the gabbro 

 and the Logan sills and their correct stratigraphic position. The gabbro 

 and the rock of the sills are petrographically the same and textural grada- 

 tions have been observed which indicate their close relationship. The 

 gabbro, while predominantly coarse-grained and granular, is locally fine- 

 grained and poikilitic, and in one place was found as a dike in the Kewee- 

 nawan and there graded into a porphyritic facies and even into a fine- 

 grained ophitic dolerite. The rock of the sills is in places, in the midst 

 of the thick sills, a good granular gabbro in texture, and ranges from this 

 through ophitic and poikilitic-textured dolerites into fine-gTained aphanitic 

 intersertal-textured basalts upon the selvage. Mineralogically they are the 

 same, excepting that in the relatively few specimens from the sills which 

 have been studied no olivine nor hypersthene has been observed, nor do 

 the sills show siich great mineralogic variation from titauiferous magnetite 

 rocks to enormous anorthosite masses, although there are small anorthosite 

 masses in the sills. Such differences in variation are, however, easily 

 explicable as due to the enormous difference existing between the masses 

 of magma forming the gabbro and that forming the individual sills. 



It is admitted by all that both the gabbro and the sills are younger 

 than the Upper Huronian, since they both have been observed at numbers 

 of places to cut the rocks of this age. The point of difference is the rela- 

 tionship between the gabbro and sills and the Keweenawan. The writer 

 found the gabbro cutting a portion of the Keweenawan rocks, whose exact 

 stratigraphic position with relation to the remainder of the Keweenawan is, 

 however, not known, and hence considers it and the sills as younger than 

 some of the Keweenawan. 



The gabbro is believed to be a great laccolitic mass which in general 

 follows approximately the contact plane between the Animikie series 

 (Upper Huronian) and the Keweenawan. In the Vermilion district there 

 are local departures from this which will be described in the following 

 paragraph. 



Over a great part of the southern edge of the Vermilion district the 

 gabbro followed essentially along the plane between the Upper Huronian 

 (Animikie) and the lower lying sediments, uplifting thereby the Upper 

 MON XLV — 03 27 



