696 THE JOURNAL OF GEOLOGY. 



possibility that we have here to do with masses which have 

 solidified at great depths. They certainly cannot, however, be 

 regarded as intrusive in the ordinary sense of the word ; so that, 

 unless we regard them as great outflows, we should be forced to 

 look upon them as the now solidified reservoirs from which the 

 ordinary Keweenawan flows have come." 1 



C. Petrographical Description of the Normal Phase of the Gabbro. 



Up to the present time there has appeared no general petro- 

 graphic description of the great gabbro supposed to be at the 

 base of the Keweenawan, although both Irving and Wadsworth 

 have given detailed descriptions of hand specimens taken from 

 it. The former writer, 2 in his monograph on the copper-bearing 

 rocks, refers to the great mass at Duluth as consisting principally 

 of a coarse orthoclase gabbro, but including some orthoclase-free 

 gabbro. The rock is "massive and irregularly jointed, making 

 great ledges facing in different directions, and furnishing bare 

 rounded summits to the hills which it composes. ): 



r The prevalent type of the gabbro ... is of a light gray color, 

 and very coarse-grained, single feldspar crystals sometimes reach- 

 ing even an inch or two in length. The augitic ingredient is 

 plainly in greatly subordinate quantity, and often on a fresh sur- 

 face its presence cannot be detected at all. On exposed surfaces, 

 however, the weathering generally brings it out, and then it can 

 be plainly seen to fill the spaces between the feldspars. Titanif- 

 erous magnetite is also often perceptible to the naked eye in large 

 particles. ); 



' Less commonly the grain is finer and the color darker, the 

 augitic ingredient at the same time becoming more plentiful. In 

 the thin section the predominant feldspar is seen to be a plagio- 

 clase belonging near the oligoclase end of the series. There appears 

 also to be a younger feldspar present, which has the character 

 of orthoclase and fills corners between the plagioclase crystals, 

 around whose contours it moulds itself sharply. Streng and 



'Copper-Bearing Rocks, p. 144. 



2 Copper-Bearing Rocks, Mon. V., U. S. Geol. Survey, p. 266 and 269. 



