60 BUIiLETDSr 184, UNITED STATES NATIONAL IVrUSEOJM 



a temperature much below 1,000°, and likewise the formation in 

 the solid state of rhabdites of perfect crystalline form in kamacite 

 plates could only have taken place at a much higher temperature 

 range than 350°-580°. All the observed structures in octahedrites 

 are consistent with the theory of then- production by slow cooling 

 from the gamma range. 



Cosmic heatings. — There is the further objection that the operation 

 of such a mechanism could be only occasional, whereas meteorites 

 of any given range of composition show a marked uniformity of 

 structure. The hypothesis of such successive reheatings is of itself 

 admissible. Certain meteors have been observed to pass at peri- 

 helion within the orbit of Mercury. A meteor would have to pass 

 considerably within that orbit to be heated as high as 350°, though 

 it may fairly be assumed that some may do so. But such cases 

 would be exceedingly rare in comparison with the overwhelming 

 proportion that move in orbits much farther from the sun. 



The structures of meteoric kons indicate uniformity in the con- 

 ditions of their production. Microscopic examination of a large 

 number of irons reveals a strikingly regular progression in structure 

 from the coarsest octahedrites to the highest nickel ataxites, each 

 type of structure corresponding with a certain range of composition. 

 Thus it seems needless to invoke the theory of a mechanism that 

 necessarily would be exceptional to explain structures that are almost 

 always present in substantially the same form within any given 

 range of nickel content. 



Transformation structure. — An important structural feature in 

 meteoric ii-ons, which apparently was first correctly interpreted by 

 Vogel, is an acicular structure produced in the gamma-alpha range, 

 which may conveniently be termed the transformation structure. 



A feature termed by him transformation figures (Umwandlungs- 

 figuren) was described by Vogel (1927) as a product of the delta- 

 gamma transformation. It appears in artificial iron-nickel alloys 

 as a network of very minute oriented needles somewhat suggesting 

 the Widmanstatten structure. He stated that it also occurs in 

 meteoric irons, mentioning as one example Smith's Mountain (pis. 

 38, 50) . Owing to the unstable and transitory character of the delta- 

 gamma structure, however, that explanation was unsatisfactory; for 

 the question naturally arises how such a structure could have been 

 preserved through the gamma range in irons of low or m^oderate 

 nickel content. 



In a later discussion (1932), however, the same author does not 

 deal with the supposed delta-gamma figures but describes at length 

 the transformation figures arising at lower temperatures in the 

 solid state. These he explains as the result of the gamma-alpha 



