18 BULLETIN 94, UNITED STATES NATIONAL MUSEUM. 



olivine with a little interstitial feldspar, and scattering granules of 

 titanic iron and chromite. The structure is comparable with that of 

 a terrestrial pyroxenite. In figure 2 is shown that of the holocrystal- 

 line siliceous portion of the mesosiderite of Estherville, Iowa, con- 

 sisting of olivine, orthorhombic and monoclinic pyroxene, and a pla- 

 gioclase feldspar. 



A very large portion of the stony meteorites consists wholly or 

 in part of rounded and oval granules called chondrules, embedded 

 in a crystalline or tuffaceous ground, and it is in these forms, both in 

 relation to structure of the spherules themselves and the ground in 

 which they are embedded, that interest chiefly centers. (See pis. 4, 

 5, and 6.) Figure 1 of Plate 4 shows the structure of a crystalline 

 chondrite from Estacado, Texas. This, as w^ill be observed, consists 

 of the rounded and irregular chondrules embedded in a crystalline 

 ground. Figujre 2 of this same plate shows a tuffaceous form from 

 Selma, Alabama, a stone consisting of chondrules in all degrees of 

 preservation down to mere fragments embedded in a tufaceous 

 ground. 



The individual chondrules occur in a surprising number of forms. 

 Borgstrom and Ramsay enumerate 19 types of composition and struc- 

 ture in the stone of Bjurbole, and it is a safe assumption that this 

 large number could be recognized in others should a sufficiently de- 

 tailed study be made. In shape they vary from almost perfect 

 spheres (pi. 5, fig. 1), often with a slight indentation on one side, 

 through oval and elongated, rarely angular (unless fragmental), 

 forms. Internally they may be of glass, crypto- or holo-crystalline, 

 with a radiate, barred, or grate-like structure, of single or many 

 individuals imbedded in a glassy or fibrous base. Occasionally the}^ 

 show a border of later formed crystals as in figure 5 of Plate 5. In 

 some instances chondrules in a more or less perfect condition make 

 up almost the entire mass of the stone as in the case of that of Allegan, 

 Michigan, or Selma, Alabama (pi. 4, fig. 2). Or, again, they may 

 be few and scattered throughout a crystalline ground, as in the case 

 of the stone of Estacado, Texas (pi. 4, fig. 1). They may be 

 so loosely attached as to fall away when the stone is broken, or so 

 firmly imbedded as to break with it. Olivine (or forsterite) and 

 pyroxene, either enstatite or a monoclinic form, are the more common 

 constituents, more rarely feldspars. A border of nickel-iron or iron 

 sulphide about a chondrule is not uncommon, the metal sometimes 

 penetrating more or less into the interior (pi. 6, fig. 1). 



Origin of the chondritic structure. — H. C. Sorby, writing in 18T7, 

 advanced the idea that the individual chondrules were originally de- 

 tached molten drops like fiery rain and their internal crystalline or 

 amorphous condition due to conditions of cooling. Eeichenbach, as 



