400 G. p. MERRILL ON METAMORPHISM IN METEORITES 



spherulitic form.s, but in no instance have I seen a chondrule or chon- 

 droidal form under such conditions as to really satisfy me of its origin 

 in situ. The occurrence of minerals of the same species under such a 

 variety of conditions as presented by both "Kugels" and ground-mass, 

 indeed, prohibits any other conclusion. 



My views have been made the subject of a previous paper^* and need 

 only brief reference here. As there stated, those chondrules of glass and 

 of cryptocrystalline and radiate enstatites are regarded as direct products 

 of the cooling of molten drops, the porphyritic forms being considered 

 mechanically derived fragments. Confining attention to these last, it 

 will be obsei*ved that they consist of phenocrysts of the various silicates 

 in a base ranging from a completely isotropic, undifferentiated glass 

 (figure 2, plate 2) through fibrous to practically holocrystalline forms. 

 The presence of this glass in the chondrules of the unaltered tuffaceous 

 forms and its absence or rarity in others is significant and will be referred 

 to later. Feldspar, if present, exists only as fragments, unless it be, as in 

 the Dhurmsala stone, a constituent of a chondrule. 



Structures common to crystalline spherulitic Forms 



Passing to the so-called crystalline spherulitic chondrites, a marked 

 difference in microscopic structure is readily observable, though there are 

 very evident traces of a one-time tuffaceous nature in the form of still 

 f ragmen tal single crystals and chondrules, as in the Bluff (figure 3, 

 plate 2), Menow, and Richmond stones. These forms are more compact 

 than the liugelchen (spherulitic) forms, and the chondrules often break 

 with the ground. Under the microscope, as in the case of the Hender- 

 sonville stone, that which seemingly represents the finely divided, almost 

 dustlike interstitial material has become converted into a finely granular 

 base in which the larger unaltered fragments and chondrules are im- 

 bedded. In my original description of this stone I say: The structure 

 is not at all that of minerals crystallizing freely from a molten magma, 

 but suggestive of a partial recrystallization of finely divided material as 

 seen in metamorphic schists. ^^ 



Further study of the slides serves to confirm me in this opinion. An 

 equally marked change is observable in the structure of the chondrules 

 themselves, which carry little, if any, true glass, but in its place a gran- 



1* Proc. Nat. Acad, of Sciences, vol. 6, no. 8, 1920, pp. 449-472. 



IB Borgsti'om makes a similar observation regarding the Saint Micbel stone, a rhodite. 

 He says : "Die structur is demgemass nicht hypidiomorphen sondern niiher sich viel- 

 mehr einer panidiomorphen Strucktur von diesselben Art wie sie in vielen kristalinen 

 Schlefen vorkommt." 



