86 THE GEOLOGY OP MINNESOTA. 



[Galibro. 



There can be but little doubt that this is the rock which was examined by 

 Streng under the name " hornblende-gabbro," and by Irving under the name " ortho- 

 clase-gabbro,"* obtained at Duluth. Streng's specimen was obtained "on the St. 

 Louis river, near Duluth, and Irving's," near the center of sec. 33, T. 50-14. Irving 

 refers to it as the "coarse gray rock of the St. Louis river bluffs, at and near Duluth." 

 From the same mass or hill range, however, Irving reports (viz., on sec. 27, T. 50-14) 

 an " orthoclase-free " gabbro. The sections of No. 1, here described, are from more 

 decayed rock than is usually found in the gabbro range. Several other numbers 

 more fairly represent it, particularly Nos. 1C, 512, 513, while No. 5 is an illustration 

 of a still further impregnation of the same rock with elements from the contacting 

 elastics. 



The relations of this rock to a series of elastics of older date are very intimate. 

 For a discussion of this association the reader may consult Part I. 



In this rock Streng proved the titanic acid to exist in the form of titaniferous 

 magnetite. The feldspar, after treatment twenty-four hours, was found insoluble, 

 and its powder, after long treatment in the same acid, gave him no jelly. He 

 concluded, therefore, that it is not anorthite, but comes near labradorite. On 

 account of the percentage of 1.61 of potash in the rock, he concluded also that some 

 orthoclase is present. 



In the hornblende he found sometimes a "viriditic" substance, giving it alight- 

 green color, with impaired dichroism, and a parallel or radiate fibrous structure. 

 In other cases the brownish-green hornblende, without viridite, is parallel-fibrous and 

 strongly dichroic. This hornblende he considered an original mineral ingredient, 

 and not a product of change from diallage. The grains of diallage did not appear to 

 him to encroach in the least upon those of hornblende, though the two were described as 

 forming a mosaic, with sharp outlines. He mentions large, black metallic, angular 

 crystals of magnetite and menaccanite, and smaller grains of the same embraced in 

 the other minerals. These, with a little epidote, chalcopyrite, quartz and apatite, 

 constitute the minerals of the rock. He concludes thus: 



" It is therefore a basic rock whose high per cent of alumina corresponds closely 

 with its abundance of triclinic feldspar. If this feldspar were anorthite the content 

 of lime would have to be higher, and that of soda necessarily less. Furthermore, if 

 this feldspar were oligoclase or andesite, the per cent of silica of the whole rock 

 would be higher, since the 1.61 per cent of potash presupposes a content of orthoclase 

 of about 9.52, which drives the silica to a high percentage. Therefore, the triclinic 

 feldspar comes nearest to labradorite. While the silica per cent rises by reason of 

 the orthoclase, it is reduced again by reason of the percentage of magnetite and 



* Eleventh Minnesota Report, p. 51. Man. v, U, S. Grl. iS"imvy, pp. 50, Si. 



