Page:The Journal of geology (IA journalofgeology11893univers).pdf/287

quite different from those of the comparatively thin flows between the sedimentary layers of the Keweenawan.

Prof. Winchell, 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 petrographic 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. Winchell 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 confine 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 different 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 Minnesota reveals a rock which is uniform in texture and composition 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