COMPOSITION AND STRUCTURE OF METEORITES 39 



2. Chondrnles of a compound, holocrystalline nature, and those 

 porphyritic through the development of olivine or pyroxene pheno- 

 crysts in a more or less glassy base are lacking in smooth exteriors and 

 though often quite spherical in outline, are as a rule more or less 

 irregular and in many instances show unmistakable evidences of an 

 origin of form through mechanical attrition. These last should be 

 designated chondroidal forms, rather than true chondrules. These 

 distinctions are well shown in Figures 3 and 5, Plate 22, and in the 

 general view from a thin section of the stone of Cedar, Tex. (fig. 2, 

 pi. 18). 



ORIGIN OF OTHER TYPES OF STRUCTURE IN METEORIC STONES 



The question of the origin of the various types of texture and 

 internal structure of stony meteorites has been, as has that of the 

 chondrules themselves, a much disputed one, as already noted. By 

 many, including such authorities as Brezina, Link, Renard, and the 

 American Wadsworth, the obscure and confused structures shown by 

 stones of the chondritic group are due merely to hasty crj^stallization 

 succeeded in some cases by crushing. To others, including Tscher- 

 mak, Sorby, Berwerth, Wahl, and the writer, they are for the most 

 part due to a tuflPaceous origin, accompanied in many instances by 

 metamorphism. That is, they are comparable with more or less com- 

 pacted and altered masses of volcanic ash or tuff. Certain stones, 

 like those of El Nakhla, Juvinas, and Shergotty, are apparently 

 products of direct cooling from a molten magma and their clastic 

 structure, when present, due to mechanical causes. Others, as those 

 of Allegan, Hessle, and Quenggouk are so plainh^ tuffaceous as to 

 seemingly be beyond argument. There yet remain certain abundant 

 types, however, belonging to what are classed as the crystalline, 

 crystalline-cliondrite, and white, gray, and intermediate chondrite 

 groups, the structures of which are obscure and which, though 

 commonly regarded as metamorphosed tuffs, yet furnish grounds for 

 reasonable doubt as to their origin. 



The writer, however, in a recent summar}", regards the tuffaceous 

 nature of the stones classed as spherulitic chondrites as no longer open 

 to argument. The crystalline types mentioned he considers products 

 of metamorphism, in this agreeing mth the other workers quoted. 

 The grounds for this belief are summarized as below. ^^ 



The most perfect chondrules and chondroidal forms are found in 

 those stones the fragmental nature of which is most pronounced, and 

 become less perfect, more highly altered, often merging imperceptibly 

 into the groundmass as the stones pass from fragmental into crystal- 

 line forms (pis. 19 and 20) as those of Estacado and Bluff and Indarch. 



" On Metamorphism in Meteorites, Bull. Qeol. Soc. Amer., vol. 32, 1921, pp. 395-416. 



