42 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



noted by Nordenskiold ^^ and others, must have taken place outside 

 of our atmosphere and in one deficient in oxygen. Perhaps attention 

 need be called to the fact that metal occurs in quantity only in stones 

 which are plainly of a fragmental nature. In achondritic types of an 

 original crystalline structure it is almost entirely lacking. It may 

 be noted, incidentally, that the average amount of metallic iron in 

 stony meteorites is 11.98 per cent, which is equivalent to 16.55 per 

 cent of magnetite or 27.16 per cent of purely ferrous lawrencite. 



The sharply angular, uncorroded condition of the silicates in pal- 

 lasites of the rockiky group has been noted as indicative of low temper- 

 ature reduction of the metal. The question may well arise, however, 

 could not this structure be produced by a shearing pressure on a 

 pallasite of normal structure in the same manner as foliated and schis- 

 tose rocks are derived from massive terrestrial forms? It seems at 

 least possible. 



Since very early in the study of meteorites, there have been held 

 radically different opinions among students as to the causes of the 

 clastic structure so pronounced a feature of many stones and particu- 

 larly those of the chondritic types. This has been referred to else- 

 where (p. 39) but it will be well to enlarge upon the matter here. 

 Attention has been called to the fact that the most perfect chondrules 

 like that shown in Figure 3, Plate 22, occur only in those rocks con- 

 cerning the tuffaceous nature of which there could be little doubt. 

 And further, that in those stones which are approximately crystalline, 

 the chondrules, where such have existed, are more or less distorted and 

 sometimes obliterated as in Plate 23. Further than this there are 

 often evident signs of compression in the mass such as has led to 

 fragmentation of certain constituents, as shown in Plate 27.^* It is 

 true that a portion of this fragmentation may be due, as has been 

 contended, to abrupt changes in temperature as when a meteorite 

 approaches the sun and then rushes off once more into the cold of 

 space, or to the shock of a collision ; but in any case they are secondary 

 and have little to do with the original structure of the stone. 



Evidences of metamorphism in which heat is the primary factor 

 are afforded by the Bereba eukrite described by Lacroix ^^ and in the 

 chondritic breccia of St. Michel described by Borgstrom ^^ who says 

 distinctly : 



Die Grundmasse is kein Verfestigungsprodukt, sender das Resultat einer 

 unvolstandigen Metamorphose eines Triimmergesteins, das aus Kornchen und 

 Splitterchen derselben Mineralien die auch jetzt den Stein aufbauen bestanden 

 hat. 



»3 Zeits. d geol. Gesell., 1881, p. 25. 



w Merrill, George P.: On Metamorphism in Meteorites, Bull. Geol. Soc. Amer., vol. 32, 1921, pp. 395-416. 



" Archiv. du Mus. d'hist. Nat., ser. 6, vol. 1, 1926, p. 35. 



»« Bull. Com. Geol. de Finland, No. 34, 1912, p. 36. 



