68 BTJL.LETIN 18 4, UNITED STATES NATIONIAL JVTUSETJM 



Segregated kamacite does not wholly disappear even in very 

 high-nickel irons. Kamacite spindles or particles are found m San 

 Cristobal (Ni-Co 26.60 percent) and even in Limestone Creek (Ni 

 29.99 percent, Ni-Co 31.47 percent). They have, however, wholly 

 disappeared in Santa Catharina (Ni 34.37 percent, Ni-Co 35.87 

 percent). In Octibbeha (Ni 62.73 percent) the mass is clear and 

 structureless, with no trace of gray darkening even with the strongest 

 etching. 



The nature of kamacite needles. — In Cape of Good Hope (Ni-Co 

 16.62 percent) there is an obscure paraeutectoid gToundmass con- 

 sisting of fine wavy and branching bodies of ligiit and dark struc- 

 tural components (pi. 22). Occasional white spindles or needles, 

 with white borders, appear in the gromidmass. Pfann referred to 

 them as kamacite bordered by taenite. Brezma considered the 

 cores to be kamacite or schreibersite. Berwerth was of the opinion 

 that they consist of schreibersite with kamacite borders and that 

 the light wavy Imes in the surrounding groundmass are kamacite, 

 while the darker component is eutectoid plessite. In the analogous 

 structure of Cowra (Ni-Co 14.25 percent; pi. 17) the spindles were 

 referred to by Brezina as kamacite, but Cohen (1905) considered 

 them to be taenite. 



The solution of this puzzling question presents an interesting 

 illustration of the value of modern metallographic methods. Cohen 

 and Brezina did not have the use of vertical illumination, and even 

 Pfann and Berwerth could not avail themselves of sodium-picrate 

 etching. When that treatment is applied to such irons, no darken- 

 ing occurs either in the spindles or in the light component of the 

 groundmass, proving the absence of schreibersite. Moreover, at high 

 magnification and with ordinary etching, the borders of the spindles 

 can be seen to merge with the lighter component, and therefore 

 must be identical with it. Perfect examples of such merger may be 

 seen in certain other irons in which the structure is clearer than in 

 Cape (Deep Springs, Tucson, Limestone Creek, Laurens County; 

 pis. 16, 20-22, 31). 



A further question arises, however, from the fact that this light 

 component in the gromidmass is not everywhere perfectly clear but 

 darkens slightly with strong etching even where the coalescence 

 begins. But the analogies of many other irons prove that taenite 

 is thus darkened, more or less, because of varying degrees of super- 

 saturation whereby the transformation of the gamma phase was 

 not quite perfect. Thus it becomes plain' that the lighter compo- 

 nent is actually identical with the clear taenite borders of the needles, 

 many small areas appearing darkened. 



The establisliment of the nature of the needles was peculiarly 

 difficult in the case of the Cape iron because the taenite borders are 



