40 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



It is therefore suggested, though not insisted upon, that the mere 

 presence of a chondrule in a meteorite, whatever its condition, is 

 indicative of a tuffaceous origin. The clear, limpid interstitial "glass " 

 sometimes quite isotropic and sometimes doubly refracting, known as 

 maskelynite, is shown to have been, together with the phosphate 

 merrillite among the last of the constituents to solidify, and probably 

 a product of a reheating and cooling too abrupt for crystallization. 

 The dark, glassy interstitial material sometimes surrounding a chon- 

 drule (fig. 1, pi. 27) and the pellucid borders presented by som.e of 

 the feldspars in the Estherville stony iron, are considered of like origin. 

 It is shown further that the occasional crushing of the individual 

 constituent, while productive of a cataclase structure, is a very minor 

 feature and without necessary bearing on the question of the original 

 nature of the stone (figs. 3, 4, 5, pi. 27). Lacroix also holds to this view. 

 This condition, it is variously conceived, may have been produced by 

 compression within the mass, by the shock of a collision, or too abrupt 

 and extreme changes in temperature, as when a comet approaches 

 the sun and then flies off again into the cold of space. It is possible 

 that all three may have operated at various times and under various 

 conditions. It is also conceivable that it may be in part due to 

 impact with the earth's atmosphere. (See further under Metamor- 

 phism). 



METAMORPHISM IN METEORITES 



Not all of the phenomena of structure and composition noted can 

 be considered original. Some are unquestionably of secondary 

 origin^ — a result of accident or changed conditions, and may in part 

 be designated metamorphic, as with terrestrial rocks. 



With the possible exception of the chondrules, no meteoric con- 

 stituent, it may be stated, presents a more interesting puzzle than 

 the metallic portion. As has been noted, this is not a simple but a 

 compound body consisting of three more or less definite allo3^s of a 

 composition, and an arrangement among themselves not found in 

 terrestrial irons, artificial or otherwise.^^ Indeed the octahedral ar- 

 rangement of the plates, when such exists, is considered sufficiently 

 characteristic to insure the meteoric nature of any iron in which it 

 may be found, whether or not seen to fall. The conditions under 

 which such an arrangement could take place are not as yet quite 

 understood and need not be considered here. The feature that now 

 concerns us is its lack of stability, its susceptibility to change, under 

 changed conditions. It has been shown that if an octahedral iron 

 like that of Toluca, Mexico, an etched slice of which is shown in 

 Figure 1, Plate 26, be heated for a few hours at a temperature below 



88 Artificial alloys comparable with the meteoric irons have been formed. (See Benedict, Neues Jahr. , 

 1912, vol. 1, p. 44). While, however, these show an octahedral structure the clearly marked separation into 

 alternating plates of kamacite and taenite is lacking. 



