Plate Ti 



1. Seneca Township, Michigan; finest octahedrite; Ni-Co 11.67 percent. A kamacite 

 band (horizontal) between fields of very coarse plessite. The kamacite shows a strongly 

 developed gamma-alpha transformation structure, producing a brilliant oriented sheen with 

 macroetching. Picral; X 60. 



2. Seneca Township. Part of the same kamacite band shown in figure 1 (here running 

 vertically) but in the zone of alteration. The acicular transformation structure is obliter- 

 ated in the kamacite, and no definite structure is recognizable. 



3. Burlington, New York; medium octahedrite; Ni 8.14 percent (Rockwell, in Silliman, 

 1844), 8.90 percent (Clarke, 1852). Taenite lamellae of a coarse plessite field much altered 

 and partly diffused by heating. Picral 50 seconds; X 60. Field Museum of Natural 

 History. 



4. Burlington. A lamella of dense plessite, altered by heat. In recrystallizing kamacite 

 flakes segregated, appearing as needles in a black unresolved gamma-alpha aggregate. 

 The surrounding kamacite shows a strong secondary granulation. Picral 50 seconds; X 150. 



Vogel (1927, fig. 11) gives a photograph of a taenite band in Burlington at lower magni- 

 fication which he describes as an example of gamma twinning preserved by rapid cooling. 

 No such structure was observed in this specimen, although man}- lamellae similar to figure 

 2 bear some resemblance to a twinning structure at lower magnification. 



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