PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 97 



Gabbro.] 



No. 5. GABBRO (ivith orthockise). 



Duluth. Near the old passenger station of the St. Paul and Duluth R. R. [The outcrop from which this 

 sample was obtained was entirely worked out and the spot covered on the construction of the new union depot; 

 but this rock is not an uncommon one at Duluth, as it is a phase of the contact phenomena and fusion of the 

 sedimentary rocks with the gabbro No. 1. It is seldom seen so coarsely crystalline, however.] 



Ref. Annual Report, ix, pages 11, 12, 19, 57. Annual Report, x, pages 41, 141. Bulletin ii, page 88. 

 American Association for the Advancement of Science, vol. xxx, page 165, 1883. 



Compare R. D. IRVING, Mon. v, U. S. Geol. Survey, page 55, 1883. 



Meg. A heavy, dark-reddish rock of rather coarse grain, composed of pinkish 

 feldspar, gray feldspar, pyroxene and magnetite. The rock has the appearance of a 

 coarse diabase, except that half or more of the feldspar has a pinkish color, and the 

 rest is gray plagioclase. Where the latter is more abundant, the pyroxene and 

 magnetite are in larger quantity; there thus appear irregular areas, not sharply 

 separated from each other, one of a pinkish, and the other of a darker color; yet, in 

 the areas occupied mostly by one feldspar, crystals of the other frequently occur. 

 There are also some grains which are intermediate in color, and a complete gradation 

 in color, from the pinkish to the gray, can be seen; moreover, in some cases, a single 

 crystal will be pinkish in one part and gray elsewhere; in such cases the centre is 

 usually of the latter color, while the margin is pinkish. The gray feldspar sometimes 

 shows twinning striae, but none were seen on the pinkish variety. Magnetite, in 

 grains of considerable size, is quite abundant; some of it yields readily to the ordinary 

 magnet, while some does not, thus indicating that at least part of what appears to 

 be magnetite is really ilmenite. In some of the areas of pinkish feldspar a few 

 small grains of quartz are to be seen. 



Mic. A basal section was made from each of the feldspars. The gray feldspar 

 in this section shows a cleavage trace, and the whole slide extinguishes at 4 or 5, 

 indicating either albite or labradorite (Fouque, page 148).* A single basal section 

 is not sufficient to decide, since it is impossible to know from which direction it is 

 examined, whether on the right or left of the obtuse angle 001:010; in other words, 

 whether it is before the observer in the " conventional position" of a triclinic crystal, 

 with the obtuse angle 001:010 at the right. In order to determine between these, a 

 careful test was made with hydrofluosilicic acid, by the method of Boricky, the result 

 being numerous large monoclinic micro-crystals of fluo-silicate of lime, as well as 

 irregular spreading and branching growths of the same, many fine hexagonal micro- 

 crystals of soda, and a few cubes of the same salt of potassium. This result precludes 

 albite and indicates labradorite, in which a small portion of the soda is replaced by 

 potash. Several measurements were made on sections nearly 010, for the extinction 

 angle, which is uniformly high, ranging from 26 to 33, this result indicating 

 labradorite or bytownite. In one of the sections examined, is a small rectangular 

 section of this feldspar, showing nearly rectangular cleavages and striations, on 



"Compare, alao: Bulletin de la Socilti Franfaise de Mineraloijie. Tome xrii, p. 428, 1894. 

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