32 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



sharp and clean as though it had been turned on a lathe. In the 

 Elm Creek example crystallization evidently began at one point on 

 the surface of the spherule and extended inward throughout, but the 

 cooling proceeded too rapidly for the production of an optically 

 perfect crystal. In the Hessle stone (fig. 4), there were evidently 

 several initial points of crystallization. Forms like these grade 

 imperceptibly into such as are shown in Figure 5, in which the radi- 

 ating bars have unmistakably the crystallographic properties of 

 enstatite. 



2. Half glassy, barred and porpTiyritic forms. — Porphyritic forms are 

 characteristics of both olivine and enstatite chondrules, while the 

 barred forms, such as are shown in the upper figures of Plate 23, are 

 limited mainly, if not wholly, to monosomatic forms composed of 

 olivine. In Figure 1, from the Beaver Creek stone, the white 

 portions are olivine which extinguish practically as a single unit; 

 the black portions are glass. It is to be noted that the outlines of 

 the chondrule though sharp are not smooth as in those described 

 above, but have projecting particles extending out into the ground; 

 also that this border portion often contains enclosures. In Figure 

 2 from the CulHson stone, the bars are bent and curved and do 

 not all extinguish simultaneously, as the stage is revolved, the dark 

 cloud sweeping over it irregularly, indicating a condition of stress. 

 Here, as in the last, the border is not sharply demarked from the 

 ground and it is often impossible to say if a certain crystal particle 

 belongs to one portion or the other. It should be noted that this 

 stone is a crystalline spherulitic chondrite. According to Tschermak, 

 in chrondrules of this nature the olivine bars are sometimes inter- 

 laminated with plagioclase (as for example, in the Dhurmsala stone). 

 In the porphyritic form shown in Figure 2, Plate 24, from the Tennasilm 

 stone, the granular ground abuts sharply against the black glass of the 

 chondrule with only on one side a manifested tendency to penetrate 

 into and beyond the border. It is to be noted that the enstatite 

 phenocrysts within the chondrule and near the border are often cut 

 off sharply as though the sphere, originally much larger, had been 

 uniformly reduced by abrasion. This will be referred to later. 



Holocrystalline chondrvles. — As would naturally be expected, these 

 porphyritic forms, through a reduction of the proportional amount 

 of glass, pass gradually into those which are almost or quite holo- 

 crystalline and polysomatic as shown in Figures 1 and 3, Plate 22, from 

 the Barratta and Elm Creek stones, respectively. Of peculiar interest 

 are those of the poly synthetically twinned pyroxene (fig. 3). For 

 some unexplained reason, these rarely grade into the half glassy 

 porphyritic forms, the entire chondrule consisting of the closely 

 crowded pyroxenes with comparatively little, if any, interstitial glass. 



