480 PROCEEDINGS OF THE AMERICAN ACADEMY 



Museum, includes the iron of Babb's Mill among those which exhibit 

 no Widmanstattian figures ; but the specimens, of unquestionable 

 authenticity, in the collection of Harvard College show well-marked 

 figures. Furthermore, an etched surface on a mass of Nelson County- 

 iron weighing 6,800 grammes, and measuring 13 inches in its longest 

 dimension, presents perfectly distinct figures near the centre of the 

 section, which grow indistinct near the edges, and entirely fade out 

 at one end. On the other hand, the iron of La Caille, classed by Dr. 

 Flight with the irons giving no figures, shows, as Dr. Brezina truly 

 remarks, very well developed figures when the sections are made 

 in most directions ; but we have etched surfaces of tliis meteorite 

 which over several square inches of area show no figures whatever. 



In view of such differences of observation and opinion as the above 

 examples indicate, it seemed desirable to make a still further study 

 of the crystalline structure of iron meteorites, and Professor Cooke 

 placed at my disposal for this purpose the very large collection of 

 Harvard College, which, since the acquisition of the collection of 

 the late Dr. J. Lawrence Smith, has representatives of all the char- 

 acteristic meteorites of this class. The writer has been familiar 

 with this collection since it was acquired by the College, and, in ex- 

 amining the specimens from time to time, he has been struck by the 

 indications of crystalline form which many of them present. These 

 appear like rough octahedrons, and their irregular faces present the 

 most striking similarity to cleavage planes, although in some cases 

 the surfaces are covered and the edges rounded by a fused crust. On 

 further examination, it appeared evident that the surfaces in question 

 must be true crystalline faces, for not only were these faces parallel 

 to planes of fracture as indicated by rifts through the mass, but they 

 also stood in a definite relation to the Widmanstattian figures. More- 

 over, rifts of fracture were common when no external crystalline form 

 could be made out, and hence came the idea of using the cleavage 

 faces as planes of reference in studying the crystalline structure of 

 the iron meteorites ; and the plan was formed to grind, jjolish, and 

 etch surfaces either parallel to the cleavage planes or in a known 

 position towards them, with the hope that it might thus be possible 

 to determine with certainty the relations of the crystalline plates 

 whose cross-sections form the Widmanstattian figures. This was in 

 general the plan of the investigation whose results are here given. 



The most striking example of such a rude cleavage form as has 

 been described is a mass of the Franklin County (Kentucky) iron 

 weighing 7,260 grammes, which shows all the planes of an octahedron. 



