76 BULLETIN 184, UNITED STATE'S NATIOKIAL MUSEUM 



phosphide being conspicuous. Forsyth County (pis. 8, 9) shows 

 similar alteration of minute phosphide bodies. An interesting fea- 

 ture of the latter iron is the presence of Neumann lines, showing 

 that the alteration was cosmic, before the meteor reached the earth's 

 atmosphere; because a degree of heat sufficient to alter the phosphide 

 would have obliterated preexisting Neumann lines. The manner in 

 which such reheating took place is speculative and belongs to the 

 domain of astronomy. 



In some hexahedrites superficial heating during flight has produced 

 zones of alteration that are virtually identical in structure with 

 nickel-poor ataxites. For example, the structure thus locally altered 

 in the Rio Loa hexahedrite (pi. 58) and the general structure of the 

 Chestervnie ataxite (pi. 9) are virtually identical, and the granular 

 structure of the Kendall County hexahedrite is similar to that of the 

 Mejillones ataxite (pi. 11). Indeed it is often a matter of judgment 

 whether a given iron should be termed a hexahedrite or a nickel- 

 poor ataxite. Even the Neumann lines are not an unfailing criterion 

 for, as mentioned above, they may occur in a nickel-poor ataxite, 

 while in some hexahedrites they are so nearly absent that they might 

 be overlooked. A cubic cleavage also would not be an infallible test, 

 for a nickel-poor ataxite, being wholly m the alpha phase, might show 

 such cleavage. 



Considering the fact that the average composition of hexahedrites 

 and of nickel-poor ataxites is practicalh^ identical (Farrington, 

 1907), and that there are abundant examples of hexahedral structure 

 changed in zones of alteration to a condition identical with that of 

 nickel-poor ataxites, we may assume that a nickel-poor ataxite was 

 originally a hexahedrite. It also could be produced by reheating 

 a coarse (low-nickel) octahedrite, and this might account for a few 

 nickel-poor ataxites with a nickel content somewhat higher than that 

 of hexahedrites. The scanty taenite lamellae in very coarse octa- 

 hedrites might be dispersed and disappear; whereas in octahedrites 

 of higher nickel content the lamellae are so strongly developed that 

 even with high heating they might not be dispersed. Thus the 

 octahedral structure would be preserved even though there might 

 be evidence of extensive alteration. Such effects, in an incipient 

 stage, may be observed in zones of alteration. 



Cosmic thermal changes. — The fact that some meteoritic structures 

 reflect quicker and others slower cooling, and still others reheating 

 in various degrees, does not involve impossible assumptions as to 

 cosmic thermal conditions. 



As stated in Chapter XI, it seems unlikely that dense ataxite 

 structures are the result of successive approaches to the sun by 

 meteors belonging to the solar system, and it has been the prevail- 



