Plate 37 



1. Kenton County, Kentucky; medium octahedrite; Ni-Co 8.49 percent. A field of 

 coarsely granular plessite. Boundaries of large gamma grains both in the field and in the 

 surrounding kamacite. Owing to strong etching there is sharp blackening around the kama- 

 cite grains, and around the small taenite threads at the edges of the field. Picral 60 seconds; 

 X 30. 



2. Be.\r Creek, Colorado; fine octahedrite; Ni-Co 7.88 percent (1867). Lamellae of 

 taenite and fields of coarse granular plessite separating kamacite bands. As is the case 

 with most coarse plessite, especially in irons showing transformation structure, there is little 

 or no taenite bordering the fields, indicating relatively rapid formation at a temperature at 

 which diff'usion was active. Even with strong etching taenite is only slightly darkened, 

 indicating fairly complete transformation. All the kamacite bands show more or less 

 transformation structure, producing parallel lines somewhat resembling Neumann lines. 

 Small schreibersite bodies are close to the taenite lamellae. 



Such plessite fields, common in the coarser octahedrites, are unusual in fine ones. Bear 

 Creek is necessarily classed as fine because the bands have a quite uniform width of slightly 

 less than 0.5 mm.; but in all its structural features it resembles a medium octahedrite. Anal- 

 yses by Smith (1867) and by Jackson (1867) give respectively 14.89 and 7.88 percent 

 Ni-Co. The former percentage is obviously impossible, but the latter would be consistent 

 with the structure. Picral 7^ seconds; X 30. 



507394—44- 



