790 THE GEOLOGY OP MINNESOTA. 



[Gneiss. 



Mic. The dark-green spots are caused by changed angite, which has given place 

 to hornblende in fine interlacing fibres, which sometimes ramify in a calcite matrix. 

 This hornblende is earlier in date than a second highly refractive mineral, which 

 occurs often in the greenstones and in the gneisses, already noted, which much 

 resembles epidote. It is light yellow in color, or almost colorless. It not only enters 

 within the hornblende mesh, filling it in a measure similar to that of calcite, but it 

 occurs as small grains within the altered feldspars, accompanying zoisite, calcite and 

 sericite.* A little apatite is seen in the feldspars. One section. 



Age. Cabotian. N. H. w. 



No. 1805. GNEISS (urith cordierite?) 



Duluth; at a point midway between the dam of the above-mentioned creek and the station of the elevated 

 railway. A large surface exposure of some old metamorphic rock. It is below the crest of the gabbro range, 

 and on the southern slope, but within the general gabbro area. Some of the rock is red and some is blue, or 

 gray, sometimes appearing like a conglomerate holding boulders of quartz and of granite. In some cases the 

 red color shades into the blue, even on the surface. 



Ref. Annual Report, xxii, page 7. 



Meg. Represents the fine-grained, gray rock. 



Mic. The slide is colored by magnetite, hornblende and by diopside, in very tine, 

 roundish grains. The remainder consists of interlocking feldspars and probably 

 with cordierite. These are all secondary minerals, due to the action of the gabbro 

 on some clastic. There is a capricious distribution of these minerals, especially the 

 magnetite, bunching up in certain areas, or being very fine or nearly absent in others, 

 indicating an irregular, perhaps a pebbly, structure in the original rock. This rock 

 resembles the gneiss of Gabemichigama lake, Nos. 1089, 1090, 1350, 1351. One 

 section. 



Age. Animikie(?) N. H. w. 



No. 1806. GNEISS ( with cordierite. ) 



Same place as the last. 



Ref. Annual Report, xxii, page 7. 



Meg. Spotted with small, green areas, appearing amygdaloidal, coarser grained 

 than No. 1805. 



Mic. The green areas are hornblendic nests. With a little biotite .and more 

 evident cordierite, this rock is not essentially different from No. 1805. One section. 



Age. Animikie(?) N. H. w. 



*In order to distinguish between epidote and diopside, as in this slide, it is necessary to find a section which lies perpen. 

 dicular to the optic plane, i. e., one in the zone of symmetry. The coloration of such a section is very different from the highly 

 colored conspicuous grains. Indeed, when the section is perpendicular, at the same time, to an optic axis, the color disappears 

 entirely, and a dull gray takes its place as viewed between the nicols. It is only by noting the high refractive index that such ;i 

 grain would be supposed to belong to the same substance. Either a bisectrix or an optic axis is always visible in sections perpen- 

 dicular to the optic plane, either in pyroxene or in epidote. When found, such an interference figure will show the direction of 

 the optic plane with respect to the longitudinal cleavage, which is then also visible. In case the optic plane is perpendicular to 

 such cleavage the mineral is epidote, as in this instance. If it is parallel with the longitudinal cleavage the mineral is a pyroxene. 

 In this case a peculiar distribution of the high colors of double refraction also shows these grains to be epidote. The same surface 

 is at the same instant red, green or yellow, this peculiarity being connected with the powerful dispersion of epidote. 



