CAUSES OF VARIATION IN STRUCTURE 405 



in temperature. This, as I view it, should effectively dispose of the idea 

 of its being an original residual glass. It is a later ' product and was 

 unmistakably the last of the silicate constituents to solidify, and, with 

 the possible exception of those involved in the deposition of the metal 

 and sulphides, represents the closing act in the series of changes through 

 which the stone has passed. 



There remains to be noted yet one other structural feature charac- 

 teristic of this group, which may be regarded as secondary and of meta- 

 morphic origin. In the Knyahinya stone (a gray chondrite) the chon- 

 drules are sometimes surrounded by a fine granular border (figure 1, plate 

 i), which seemingly represents the metamorphism of an interstitial dust- 

 like material. The same feature, in a more pronounced form, is shown 

 in sections of the Mezo Madaras, Parnallee (figure 2, plate 4), and Eich- 

 mond stones, the first two gray chondrites and the last a crystalline 

 spherulitic form. I am convinced that this is likewise of a secondary 

 or metamorphic nature, a product of heating sufficient to affect (sinter) 

 the finer interstitial material, but not sufficient to obliterate the original 

 fragmental structures. I will again refer to this in discussing the agen- 

 cies of metamorphism. 



Agencies of Metamorphism 



Assuming the features above described are of secondary origin and due 

 to metamorphism, the question arises as to how this metamorphism has 

 been brought about. Before entering on a discussion of this it will be 

 well to again call attention to the entire lack in the meteorites of such 

 secondary and more or less hydrated minerals as are common charac- 

 teristics of metamorphosed terrestrial rocks. 



♦ The field of speculation is at once narrowed, though not necessarily 

 simplified. It can be safely stated, however, at the outset, tlrat the 

 changes noted have taken place in an atmosphere free from moisture and 

 influences promoting oxidation. Evidently they were limited to dry 

 heat and pressure. 



Perhaps one of the most striking features of the chondritic meteorites 

 is their paucity, outside of the chondrules, in a residual, undifferentiated 

 glass base, and it is a most singular fact that those who advocate the 

 "hasty crystallization" origin of these stones have failed to realize a con- 

 dition so fatal to their conclusions. 



I do not agree with Weinschenk and others, that in certain stones like 

 Chassigny, Ensisheim, Farmington, Mezo Madaras, and Parnallee, glass 

 is so abundant as to form a netw^ork in which the other minerals are im- 



