THE METAX,L0GR1APHY OF METEORIC IRON 93 



by impact upon the earth. It cannot be disproved, of course, that 

 deformations and disruptions may not have taken place outside of 

 the earth's atmosphere. The pressure of the air during flight, how- 

 ever, is of such extreme intensity that the resulting stresses would 

 seem to be sufiicient to account for such phenomena. The action of 

 the air in such circumstances is comparable in intensity (though not 

 in suddenness) with that of a charge of high explosive, resulting some- 

 times in the production of large fissures and even in the complete 

 disruption of the mass. 



Evidence of air pressure. — This conclusion is supported by direct 

 evidence in the case of iron meteorites (e. g., Pitts), that give clear 

 proof of having broken in the air. The fragments of Pitts, which 

 were observed to fall, have surfaces furrowed and sculptured by 

 heat and air pressure during flight, and also freslily fractured sur- 

 faces showing no alteration; thus proving that disruption occurred 

 after the flight through the air had been partly completed. Stony 

 meteorites aft'ord many such examples, fragments showing surfaces 

 in all stages from a heavy fusion crust to a slight scorching, or 

 a freshly broken and unaltered surface. 



In lenticular masses that traveled flatwise through the air (e. g., 

 Seneca Township, Wood's Mountain), the author (1939) found the 

 cracks to be almost entirely in the peripheral portions and to enter 

 the mass only on the rounded side that was foremost in flight — the 

 pressure tending to bend backward the thinner peripheral portions 

 and thus cause the mass to open on the forward side, but to compress 

 the rear side where no cracks appear. 



The example of Helt Township is of interest because the partial 

 granulation produced by heating during flight through the air was 

 evidently contemporaneous with or subsequent to the displacements, 

 thus proving that the latter could not have been caused by impact. 



XVII. METALLOGRAPHIC TECHNIQUES 



Though metallography utilizes many methods of chemical and 

 physical research, it is impracticable in this chapter to do more than 

 give an outline of those found most useful in the study of meteoric 

 irons. These include the preparation of specimens, macroexamina- 

 tion, microexamination, photography, and the identification of 

 components by chemical tests, the spectroscope, X-ray analysis, 

 and other methods. 



Specimens for study. — Meteorites of any type are so rare that it is 

 difiicult to obtain specimens except by purchase from a reliable dealer. 

 Small samples are usuaUy sufficient and are much more convenient 

 than large ones. In the case of octahedrites, because of their coarse 



