THE METALLOGRAPHY OF METEORIC IRON. 73 



heating, which already had extended throughout the mass. In 

 those nickel-rich ataxites in which the structure consists of a very 

 fine gamma-alpha aggregate, no noticeable change would be likely 

 to occur. In Freda, which has a marked acicular gamma-alpha 

 structure, that appearance is obliterated in a very narrow zone. In 

 Morradal the paraeutectoid two-phase structure, very perfect and 

 uniform in the interior of the iron, becomes coarser near the surface 

 by the agglomeration of the minute kamacite and taenite areas; 

 the author, however, doubts whether this difference is due to 

 superficial heating. 



Extent of heating. — Bjr reason of the short exposure to the heat 

 generated in atmospheric flight, and the extremely cold interior of 

 the mass, the depth of alteration in such zones is slight. The alter- 

 ation is sufficient to obliterate Neumann lines, which disappear with 

 even very short heating in the range from 800° to 1,000°, and to 

 produce a definite granulation of kamacite. Both changes can 

 usually be observed in an incipient stage near the inner boundary 

 of the zone. The heating usually is insufficient anywhere to alter 

 even thin taenite lamellae, or to cause more tlian incipient diffusion 

 in plessite fields. 



These observed effects are consistent with the results of experi- 

 ments by various investigators that have indicated that a sojourn 

 of a few seconds at around 1,000° would produce such changes, but 

 that a more prolonged or higher degree of heating would cause more 

 extensive alteration. 



Fusion crusts. — The superficial heating of an iron meteorite during 

 its flight through the an- probably always produces a coating of 

 magnetic oxide (ferrosic oxide, FeO.Fe203, or Fe304), which often 

 remains as a fusion crust on irons that have recently fallen, or that 

 have lain on the surface of the earth in arid regions. Such a crust 

 is black, slightly shining when fresh, and usually very thin, com- 

 monly not exceeding in thickness heavy paper or thin cardboard. 

 Occasionally it has a slaggy appearance, and sometimes it exhibits 

 a flow or drift structure of fuie striae or ridges. 



The substance of the fusion crust, which also penetrates cracks in 

 the mass, was called "Eisenglas" by Reichenbach and that term 

 was somewhat used by the older writers. It is essentially ferrosic 

 oxide, although it may to some extent be a mixture of ferric and 

 ferrosic oxides, and it may also carry in solid solution a little sulphide 

 or phosphide derived from inclusions of those substances. Large 

 nodules of troilite thus melted out may have produced, or started 

 the production of, the rounded cavities so conspicuous on the surface 

 of Canyon Diablo and some other irons. 



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