Plate 77 



1. New Baltim<jre (see pi. 76). Typical structure at high magnification. The peculiar 

 appearance, suggesting cold working at lower magnification, is shown to be a gamma-alpha 

 transformation structure. Picral 90 percent, nital 10 percent, 15 seconds; X 600. 



2. New Baltimore. An area similar to the last, X 1,200. The nature of the struc- 

 ture is more apparent. Although the presence of such a structure might seem anomalous 

 in an iron with only 6.74 percent Ni-Co, which should be almost wholly in the alpha 

 phase, still analogous structures are found in irons with still lower content — e. g.. Primitiva 

 (pi. 10) with 5.61 percent Ni-Co, and San Francisco del Mezquital (pi. 78) with a Ni-Co 

 content of 6.33 percent. The apparent explanation is that the fine gamma-alpha structure 

 was a result of rapid cooling, analogous in its manner of formation to martensitic structures 

 in artificial irons. 



3. Salina, Utah; medium octahedrite; no analysis. This iron has been changed to 

 limonite by weathering. Traces of the original octahedral pattern are discernible. Un- 

 etched; X 1}^, central illumination. 



4. Grant, New Mexico; medium octahedrite; Ni-Co 9.98 percent. A silicate inclusion 

 in troilite. The surface nf the latter was chipped in polishing. Picral 60 seconds; X 30. 



197 



507;«>4— 44- 



