92 BUIiLETiN 184, UNITED STATES NATI0!NIA1. MUSEUM 



"False" Neumann lines. — Despite the fact that the appearance of 

 Neumann lines is so characteristic, certain structural appearances 

 of very different origin may sometimes be mistaken for them. That 

 is especially true in kamacite showing a gamma-alpha transformation 

 structure, which may show striations that at moderate magnification 

 resemble indistinct, and even fairly clear, Neumann lines. Examples 

 are illustrated in plate 35. With high magnification the illusion is 

 quickly dispelled, the nature of the structure being apparent. 



Very fine oriented needles of schreibersite, as in Walker County 

 (pi. 52), might also have such a resemblance. 



Deformations. — Some meteoric irons bear evidence of deformations 

 subsequent to the establishment of their structure, varying from 

 large cracks or complete disruption down to very minute 

 displacements. 



Microscopic displacements have been observed by various inves- 

 tigators. An outstanding example is El Inca (Tamarugal) described 

 by Rinne and Boeke (1907), in which a troilite nodule was broken in 

 two, and one half slid past the other. Helt Townsliip shows dislo- 

 cation of taenite lamellae (pi. 76) and similar appearances have been 

 described in Puquios by Brezina (1895) and by Cohen (1905) in 

 Chesterville. Striking examples of displacements are found in Canyon 

 Diablo (pi. 75) and New Baltimore (pi. 76). 



Causes of displacements. — Various causes have been suggested for 

 such displacements, the most frequently mentioned being the shock 

 resulting from the impact of the mass upon the earth. Rinne and 

 Boeke make that suggestion in the case of El Inca. They also men- 

 tion possible cosmic causes, as well as the disruptive force developed 

 by the expansion of troihte at its beta-alpha transformation, which 

 takes place at 130°, pointing out that cracks have been observed 

 near large troilite inclusions. 



The last explanation is clearly inapplicable to the case of El Inca, 

 because the marked displacement of the two halves of the troilite 

 nodule shows that it was broken by an outside force. In any view, 

 the expansion of a troilite nodule would be small in comparison with 

 the bulk of the surrounding iron, and it would be exerted equally 

 in all directions; so as a disruptive agency it would be negligible. 

 Moreover, displacements — to the extent of wide cracks and fissures — 

 are found in irons containing little or no troilite. 



The shock of impact also is an unsatisfactory explanation, because 

 often the impact is not violent. In the case of Helt Township, for 

 example, the small mass fell with so little force that it only slightly 

 buried itself in soft earth. 



In New Baltimore (pi. 76) the extensive distortion seems to be 

 the result of stresses greater than would likely have been produced 



