Plate 26 



1. Smithland, Kentucky; nickel-rich ataxite; Ni-Co 17.36 percent. Typical structure, 

 a fine segregation of very minute alpha (kamacite) crystallites in a gamma matrix. A 

 phantom pattern of former large gamma grains whose boundaries have disappeared. Picral 

 80 seconds; X 60. U. S. National Museum. 



2. Smithland. The same structure, 40 seconds picral; X 150. The structure could not 

 be well established at any higher magnification, though other etchants were used, including 

 nitric-acetic acid, which caused heavy pitting in the kamacite particles. The appearance 

 suggests an undercooled gamma structure, resulting from relatively rapid cooling to a range 

 where diffusion was sluggish, producing a fine dispersion of alpha particles. 



3. Babb's Mill (Troost iron), Kentucky; nickel-rich ataxite, Ni-Co 18.97 percent. The 

 general structure, with a kamacite needle. Picral 45 seconds; X 60. American Museum 

 of Natural History. 



4. Babe's Mill. The same structure at high magnification; an irregular paraeutectoid 

 of taenite in kamacite, the taenite slightly darkened by reason of supersaturation. Picral 

 25 seconds; X 600. 



146 



