THE BASIC MASSIVE ROCKS, ETC. 713 



quite different from those of the comparatively thin flows 

 between the sedimentary layers of the Keweenawan. 



Prof. Winehell, in his bulletin on The Iron Ores of Minnesota, 

 asserts 1 that the " gabbro is found associated with red syenite, 

 quartz-porphyry and various sedimentary rocks in northeastern 

 Minnesota, and, indeed, it passes through unimportant petro- 

 graphic changes into the well known ' traps ' of the cupriferous 

 formation, from which it has not yet been possible to separate it 

 by any important lithologic or stratigraphic distinctions." But 

 since Prof. Winehell has included within his gabbro the rocks of 

 Bellissima Lake, Carlton's Peak and the feldspar masses enclosed 

 in the dark trap of Beaver Bay, it is plain that he does not con- 

 fine his remark to the rock to which the writer is now limiting his 

 attention, viz., the great coarse gabbro which Irving described 

 as the great basal flow of the Keweenawan. This rock, as has 

 been shown, by a study of specimens taken from very many diff- 

 erent localities (see list of specimens studied, p. 714) within the 

 area underlain by it, is so very uniform in its characteristic 

 features that no difficulty is experienced in distinguishing its thin 

 sections from those of any, other rock in Minnesota north of 

 Lake Superior. 



Summary. — The microscopical study of the gabbro of 

 Irving's " basal flow " at the bottom of the Keweenawan in Min- 

 nesota reveals a rock which is uniform in texture and composi- 

 tion throughout its entire extent. It is composed of magnetite, 

 olivine, diallage and labradorite as essential constituents, with a 

 little biotite and occasionally a very small quantity of quartz 

 as secondary components. Its structure, or better texture, is 

 typically granitic in that all of its comprising minerals are 

 hypidiomorphically developed, with the plagioclase younger than 

 the diallage. In this respect the rock is essentially different 

 from the so-called gabbros of the thick flows interbedded with 

 the clastic beds of the Animikie series and the Keweenawan 

 group in the same region, for in the latter, notwithstanding the 

 1 L. c, p. 124. 



