204 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. XIII. 



Analysis by 0. Burger gave the following: 



Fe Ni Co Cu Cr P S 



54.62 4.79 0.60 0.04 trace 0.18 trace =100.22 

 The Smithsonian iron was described by Shepard 5 in 1881 as an iron of an unknown locality in the old Smithsonian 

 Museum at Washington. He observed on an etched surface a uniformly oriented sheen and concluded, therefore, that 

 the mass was an individual. A concealed banding and the appearance of compact shining flecks gave the iron a peculiar 

 structure. There was no formation of iron chloride. His son gave the following analysis (specific gravity, 7.589): 



Fe Ni Co Cu Schreibersite 



92.92 6.07 0.54 trace 0.56 =100.09 



Brezina 18 united the Smithsonian iron, on account of its resistance to acids and its richness in rhabdite, with Fort 

 Duncan. He also mentioned upon it well-marked etching pits and very fine Neumann lines, some systems of which 

 were continuous, others pinnately developed on long striae parallel to one another. 



Fort Duncan was first mentioned in 1886 simultaneously by Hidden 7 and Brezina. 8 According to Hidden 7 the 

 mass weighing 43.5 kgs. was found by Cusick on an old river terrace on the Rio Grande in the neighborhood of Fort 

 Duncan. It had the form of a flattened ellipsoid and was covered with a thin, generally dull, somewhat blistered arust. 

 One face showed large depressions and small indentations. On moderate etching two systems of lines appeared which 

 could be referred to twinning lamellae. On stronger etching these disappeared, and lines consisting of minute schrei- 

 bersite lamellae, arranged according to different directions but comparable in appearance to quartz in pegmatite, 

 appeared. For recognition of this structure magnification was necessary. To the naked eye the etched surface 

 appeared weakly spotted. Hidden noted an abundance of troilite and schreibersite in his preliminary note, but in 

 his later description, mentions only two short fissures filled with graphite and a small nodule of troilite. He consid- 

 ered Fort Duncan similar to Auburn, Hex River, and Lick Creek, but different from Coahuila and Sanchez Estate. 

 He gave the following incomplete analysis by Mackintosh: 



Fe Ni+Co P 



94.90 4. S7 0.23 =100 Specific gravity =7. 522 



Brezina 8 determined Fort Duncan to be a hexahedrite, noted its unusual resistance to acids, its richness in 

 rhabdite greater than that of any other iron, its similarity with Sanchez Estate, and a scarcity of troilite. In 1895 he ls 

 mentioned in Fort Duncan Reichenbach lamellae 8 cm. in length in the neighborhood of which the Neumann lines 

 were lacking. He also noted hemispherical pits on the surface, due to the melting out of troilite, and bent edges on 

 the point of impact of the mass. 



I have noted above, under Sanchez Estate, that Huntington 9 at first considered Fort Duncan, Sanchez Estate, and 

 the Butcher irons, Coahuila, as belonging together, but later " he separated the first two from the last. 



Meunier 10 noted the similarity of Fort Duncan to Braunau, likewise an abundance of troilite, some of which was 

 surrounded by graphite containing bands of daubreelite and showing polygonal boundaries. He separated Fort Duncan 

 from Coahuila; the former he placed in the group of Braunite, but the latter he regarded as forming a single group, 

 Coahuilite, which is distinguished by difficult solubility in acid. Brezina made an exactly opposite observation. 

 Meunier 10 > 15 furnished two incomplete analyses as follows: 

 Fe Ni Residue 



92.02 6.10 1.80 =99.92 Specific gravity =7. 699 



91.90 7.03 =98.93 Specific gravity =7. 72 



In 1889 I published an analysis 12 as follows: 



Fe Ni Co P Residue 



92.58 6.66 0.73 0.28 0.01 =100.26 



As I was later in doubt of the correctness of this analysis I made a further new investigation, and the following 

 new analysis by Hildebrand resulted (specific gravity, 7.8437): 



Fe Ni Co Cu Cr P S Residue 



94.65 4.82 1.07 0.04 0.04 0.23 0.32 0.02 =101.19 

 Linck I4 found the sheen oriented according to at least five different planes of 112 and concluded that the brilliancy 

 of the orientation was dependent on the breadth and abundance of the twinning lamellae. When he later recognized 

 that etching pits also contributed to the oriented sheen he did not mention Fort Duncan especially but must have 

 intended to modify his view in general. 



According to the above investigations Sanchez Estate, Saltillo, and Fort Duncan masses have in common an 

 unusual resistance to etching with nitric acid and to solution by hydrochloric acid. Most iron meteorites dissolve in 

 very dilute hydrochloric acid (lHCl+20aq.), but a piece of Sanchez Estate weighing 35 grs. lay 8 days in such a solution 

 unchanged, and even an acid twice as concentrated as this acted upon it very slowly. Fort Duncan will not receive a 

 complete polish, large portions remaining rough. In these places under a lens can be distinguished closely crowded 

 minute needlelike or short linelike depressions which run parallel to one another. These seem to originate in small 

 rhabdites, which on account of their brittleness break out under polishing. In the same way as found by Brezina for 

 Sanchez and the Smithsonian iron Fort Duncan shows numerous very fine and very long etching lines first plainly 

 distinguished under the lens. Occasionally they are crowded together in great number; in other places they lie isolated. 

 Shorter etching lines are not lacking but are much rarer than in other hexahedrites and are only recognizable by careful 

 study of an etched surface. Besides the small rhabdites which are often first made known by the above-mentioned 



