AGENCIES OF METAMORPHISM 411 



vanced, considering ttie manner of growth. Tlie laminated broncit (netz- 

 broncit), acting as a cementing substance, is crystallized from a fused magma, 

 and therefore the thought of a secondary cataclastic structure caused through 

 pressure is entirely eliminated. The existing cataclastic disruptions can be 

 sufficiently explained by a sudden change of temperature to which the meteoric 

 substance was exposed when in this process of melting." 



The objections of both Renard and Berwerth, as it seems to the writer, 

 are done away with if we consider the shattering as incidental to the 

 general compression to which the stones have been subjected, as having 

 taken place prior to the formation of the netzbroncit and, above all, to 

 that of the maskelynite. It must be recalled that neither of these minerals 

 show any trace of mechanical distortion, miless it may be that indicated 

 by a slight optical anomaly. The maskelynite in no case that has come 

 under my observation shows any trace of the fracturing so constant a 

 character of the olivines and pyroxenes. It was obviously the last min- 

 eral to solidify, and its production was the closing act in the series of 

 changes and conditions to which the stones have been subjected. 



Although the interpretation I have put in the foregoing pages on cer- 

 tain structural details may in some cases seem open to question, never- 

 theless the facts as given, it is thought, fully substantiate the idea of 

 metamorphism in meteorites — that the crystalline forms were derived 

 from the tulfaceous, rather than the reverse. Whatever lingering doubts 

 one may have are due mainly to the lack of certain incidental effects 

 which seemingly should have been manifest. The most striking of these 

 discrepancies is the small amount of true magmatic glass found in any of 

 these forms. It would seem strange that were this metamorphism due so 

 largely to dry heat, as is apparent, occasional forms should not be met 

 with that are wholly vitreous or that, at least like some of our modern 

 lavas, are largely glass with varying proportions of phenocrysts. Any 

 argument based on this feature may be used, however, with equal force 

 against the idea of direct cooling from a molten magma. The suggestion 

 that the so-called tektites may be representatives of the missing glassy 

 forms is ruled out on chemical grounds. 



A brief summary of the matter in the foregoing pages may be set forth 

 as below: 



I. Evidence of metamorphism mainly through dry heat. 



(1) The presence of interstitial maskelynite and merrillite. 



( 3 ) The "glass" border about the kugel. 



(3) The absence (in the metamorphosed forms) of glass, other 

 than maskelynite, in the chondrules and the presence in its 

 place of fine granular crystalline matter. 



