METEORITES — HENDERSON AND PERRY 239 



ceases and it therefore is no longer luminous. During the fall heat 

 diffuses slowly into the mass, so that the oulsidc rapidly increases in 

 temperature. Soon the surface is softened and removed by the 

 scouring action of the air. This type of erosion progresses with the 

 jienetration of heat. As the falling mass is constantly entering a 

 denser atmosphere, the frictional resistance increases; consequently, 

 the stripping away of the outside goes on at an accelerated rate. 

 The size of the falling body is reduced rapidly. 



It has been demonstrated that the symmetrical etch pattern of a 

 cross section of an iron meteorite is easily and rather completely dis- 

 rupted by heating to a bright red heat. So if heat has penetrated 

 deeply into these irons their interiors would be found without this 

 formal structure. Examinations of scores of cross sections of irons 

 have been made, and no evidence exists to show that the interior was 

 heated during the flight. The heated zone detected by study of sec- 

 tions is seldom more than 1 or 2 cm. thick. 



The surfaces of many stony meteorites show well-defined markings 

 that indicate the orientation of the body through a portion of its 

 flight. Since only a few iron meteorites have been seen to fall, most 

 of those now known are old ones on which the flight markings have 

 been removed by alteration. The finding of such a perfectly pre- 

 served set of flight markings as the Freda, N. Dak., iron is unusual. 



The outer surface of this specimen tells the story of its struggle 

 through our atmosphere. The Freda iron is small, weighing only 268 

 grams, but its shape and internal structure make it of great interest, 

 and its flight markings are beautifully preserved. The nose of this 

 specimen is turned over just as though it had been hammered. There 

 is no reason to suppose that this resulted from its impact with the 

 ground since it was found in soft sod ; hence it appears that this rolled 

 edge is due to the resistance offered by our atmosphere. The turned- 

 over nose must have developed during the last portion of the iron's 

 flight, because had it developed very high above the earth, it would 

 have been stripped off before the meteorite landed. This mass must 

 have attained a very high velocity to produce air pressure capable 

 of turning or battering the metal in such a fashion. A determi- 

 nation of the composition of this meteorite showed that it contained 

 23.49 percent nickel and 75.86 percent iron. Material of such a compo- 

 sition would not change from the solid to the liquid phase until it 

 reached a temperature of about 1470° C. It would not be necessary, 

 however, to attain that temperature to soften the metal to a point 

 where it could rather easily be washed off by the air stream. The tem- 

 perature required to accomplish this would probably be several hun- 

 dred degrees lower than the melting point. 



In addition to the more delicate lines on the surface of meteorites, 

 a number of depressions are usually present; some are shallow and 



