COARSE GABBROS OF THE ST. LOUIS RIVER. 269 
inclination of the mass as a whole; judging from the adjacent rock beds 
this is some 45° southeastward from the western extremity to near Duluth; 
about the same amount eastward at Duluth, and a good deal less than this 
to the south of east on the Cloquet. No sign of anything like subordinate 
bedding can be seen in the rock itself. It is massive and irregularly 
jointed, making great ledges facing in different directions, and furnishing 
bare, rounded summits to the hills which it composes. 
The prevalent type of the gabbro of this belt and the kind constituting 
the hills at Duluth is of a light-gray color, and very coarse-grained, single 
feldspar crystals sometimes reaching even an inch or two in length. The 
augitic ingredient is plainly in greatly subordinate quantity, and often on a 
fresh surface 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 left between the feldspars. Titaniferous magnetite is 
also often perceptible to the naked eye in large particles. 
Less commonly the grain is finer and the color darker, the augitic ingre- 
dient at the same time becoming more plentiful. In the thin section the 
predominant feldspar is seen to be a plagioclase belonging near the oligo- 
clase end of the series. There appears also to be always a younger feld- 
spar present, which has the character of orthoclase and fills corners between 
the plagioclase crystals, around whose contours it moulds itself sharply. 
Streng and Kloos found 1.61 per cent. of potash in the rock, which they 
very properly regarded as belonging to orthoclase. The spaces between 
the feldspars are filled with a diallage which is always more or less altered to 
greenish uralite. The alteration in many sections is carried beyond uralite, 
to chlorite. The magnetite is very large, abundant and titaniferous. Apa- 
tites of large size are found in all sections. Biotite is a not uncommon 
accessory. Olivine is absent from all sections. A large-sized figure of the 
thin section of the Duluth rock is given on Plate VI. 
1 Microscopic descriptions of the Duluth gabbro have been hitherto published by Streng (Ueber 
die Krystallinischen Gesteeine von Minnesota in Nordamerika—Leonhard u. Geinitz, Neues Jahr- 
buch fiir Mineralogie, Geologie und Paleontologie, 1877, p. 113), and N. H. Winchell (Eighth An- 
nual Report of the Geological and Natural History Survey of Minnesota, 1880, p. 22). Streng calls 
the rock ‘‘hornblende-gabbro,” regarding the hornblende as primary. His conclusions are summed 
up as follows: ‘‘The hornblende-gabbro of the Saint Louis River at Duluth consists of a greatly pre- 
