90 BTJULETDvi 18 4, UNITED STATES NATIOKIAL MUSEUM 



eral, are conditioned by the strength, direction, or suddenness of the 

 deforming force, and also no doubt by the structural properties of 

 the iron. That one set of lines should be very fine while another 

 in the same area is broad would be consistent with such a genesis. 

 The occasional exaggerated breadth of a given set, usually with 

 irregular or wavy boundaries, would naturally result from the sec- 

 tion's having been made at an angle oblique to the twinning plane; 

 whereas if made at right angles to it, the lines would appear finer 

 and more sharp-cut (pi. 7) . 



Differences in the prominence of difterent sets of lines may also 

 in some cases be due to the more active attack of the etchant along 

 certain planes than along others. According to Stead, cubic planes 

 are more resistant than octahedral planes. 



Smith and Young (1939) suggest that slip planes are due to dif- 

 ferential contraction in cooling and to expansion stresses caused by 

 an increase in volume at the point of the gamma-alpha transforma- 

 tion. Neumann lines, however, are readily produced in cool, or 

 cold, artificial iron; and the fact that they are found in Forsyth 

 County and Dungannon, which apparently were reheated strongly 

 enough to obliterate any previously formed lines, would indicate 

 that they arise at temperatures below the point of the gamma-alpha 

 transformation. 



Resemblance to gamma twinning. — The appearance of gamma 

 grains due to twinning in a high thermal range, though somewhat 

 resembling Neumann lines, is different in character. The gamma 

 grains in artificial iron are marked by profuse twinning. It has 

 been suggested that this might be due to stresses caused by sudden 

 increase in volume at the point of the gamma-alpha transformation, 

 but heat etching in the gamma phase has proved the existence of 

 the twinning structure in that range. 



Neumann lines in artificial ii-on are produced only in alpha ferrite, 

 the more readily the cooler the metal is. In meteoric irons they 

 occur only in alpha kamacite. 



Manner of formation. — The Neumann lines in meteoric iron are 

 clearly the result of stress or shock in the alpha phase, like those 

 produced in that manner on a microscopic scale in the grains of 

 ferrite. 



In the literature of meteorites little is found regarding the nature 

 or origin of the shock or stress whereby the Neumann lines could 

 have been produced. To the author the most probable cause would 

 seem to be the violent stresses caused by atmospheric pressure during 

 flight. This is discussed later in this chapter. 



As pointed out by Rosenhain and McMinn (1925) the tendency 

 of a blow is to concentrate the slippmg in a few broad bands, while 

 gradual deformation distributes it over a number of planes. Thus 



