74 BULLETIN" 184, UNITED STATES NATIONIAL IVrUSEUM 



Intrusive oxide. — The black substance filling cracks extending 

 from the surface toward the interior of many irons was early recog- 

 nized as identical with that of fusion crusts. In such cases it extends 

 to the uttermost limits of the cracks — as might be expected, inas- 

 much as it was forced into them by extreme air pressure as fast as 

 the cracks themselves were formed. 



In octahedrites the oxide may invade the structure to some extent, 

 especially near the inner terminations of the cracks, penetrating for 

 short distances along kamacite bands. It is also apt to follow or 

 invade inclusions of schreibersite because of its low fusibility. 



In appearance under the microscope magnetic oxide and hydroxide 

 are often indistinguishable, but the intrusive oxide usually can be 

 recognized by the manner of its occurrence. 



An area of intrusive (magnetic) oxide resembles a bay or river, with 

 rounded and clear-cut boundaries. All the normal structures within 

 it are wanting, while the structure m the adjoining mass is undis- 

 turbed. In cracks the oxide sometimes shows structures resulting 

 from quick cooling (pi. 69). 



The hydroxide, on the other hand, invades the mass irregularly, 

 occupying individual grains or penetrating along grain boundaries. 

 Taenite particles in plessite may be unaffected, and taenite lamellae 

 often remain unchanged, preserving perfectly the outlines of the 

 octahedral structure (pi. 69). The hydroxide also may surround 

 perfect rhabdites, which would have been melted if the oxide were 

 intrusive. 



Dendritic crystallization.- — A unique eflFect from the action of 

 intrusive oxide was observed by the author in Seneca Township 

 (1939) and in Wood's Mountain (1939), in both of which the oxide 

 fills small depressions in the surface and encloses minute rounded 

 areas of iron exhibiting a beautiful dendritic crystalhzation, a struc- 

 ture which it is believed had not previously been observed in meteoric 

 irons (pi. 70). The author also found such areas in Carlton and 

 Reed City. 



As dendrites are produced by rapid cooling from the melted state, 

 they could not exist as a primary structure in the mass of an iron 

 meteorite. It is evident that the iron was invaded slightly at cer- 

 tain points by the superheated liquid solution of iron and ferrosic 

 oxide formed on the surface during flight, which accumulated on 

 the rear side where it remained long enough to penetrate slightly at 

 favorable spots. The curvature of the octahedral structure around 

 a number of such spots suggests the former presence of inclusions of 

 troilite, which would have melted quickly and thus facilitated such 

 invasions. 



As the oxide-iron solution quickly cooled the oxide was rejected, 

 the iron separating in rounded droplets, which developed a dendritic 



