COMPOSITION AND STRUCTURE OF METEORITES 33 



In Figure 1, Plate 24, from the Parnallee stone, it will be noted that 

 the crystals are in some instances slightly curved, their vertical axes 

 lying approximately parallel with the circumference of the circle 

 which forms the border of the section. The appearance is as if the 

 chondrules had been molded by external forces after the crystals had 

 formed but while yet in a more or less plastic condition. Again, the 

 pyroxene crystals abut sharply against the border and are cut off at 

 the margin as in the half glassy, porphyritic forms mentioned above, 

 and as shown in the figures. Occasional forms are met with which 

 have all the appearance of fragments, slightly rounded, of holo- 

 crystalline granular rocks, which as noted later, they are believed 

 to be. 



Secondary borders about chondrules. — A not uncommon feature of 

 the chondrule is the narrow border or rind about the circumference. 

 These borders as a rule, are of lighter color than the interior, of a 

 clear, more pellucid nature, though it may be including portions of the 

 minerals characteristic of the matrix in which they are embedded. 

 This is well shown in the olivine chondrule. Figure 1, Plate 23. This 

 border has an appearance at once suggestive of the secondary inter- 

 growth or enlargement often seen in feldspars and other minerals of 

 terrestrial rocks. The later portions sometimes, though not always, 

 have the same optical orientation as the interior.^^ In some instances 

 the chondrules are surrounded by an irregular border of metal or 

 metallic sulphide. 



Double or compound chondrules. — Occasional forms are met with in 

 which a large crystal of olivine or pyroxene is inclosed by a border of 

 finer crystals of the same mineral but suggestive of a later generation. 

 Of greater interest is the occasional occurrence of a chondrule within 

 the mass of a second or larger form, as figured by Tschermak, on 

 Figure 1, Plate 8, of his Beschaffenheit, 



Theories of origin. — In this review it will perhaps not be necessary 

 to go back much beyond the time of the introduction of the micro- 

 scope and thin sections into the study of rock structures since obvi- 

 ously little that was accurate could be told of them by the naked eye 

 alone. 



A brief glance at the literature is sufficient to suggest that many 

 of the opinions expressed have been based upon examinations of 

 but a limited number of occurrences which quite failed to yield 

 the information necessary for buUding satisfactory hypotheses or 

 conclusions. 



Reichenbach, as early as 1860 wrote: ^^ "Aus allerdem wird es 

 klar, das die Einschlusse in dem Meteoriten, als die Triimmer und 



•• I am not certain if this border is of a like nature to that described by Tschermal£ about some of the 

 chondrules of the Qrosnaja stone and which he considered of secondary origin. 

 e« Pogg. Ann., vol. 3, 1860, p. 384. See also Chamberlin's The Two Solar Families. 



