182 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1917. 



As will be noted, these show very small amounts of alumina as 

 compared with pyroxenes found in terrestrial rocks. The mineral 

 from Busti compares closely with diopside in composition, while that 

 of Shergotty is more nearly that of hedenbergite. 



The form and internal structures of meteoric masses are no less 

 interesting than their composition. The crj^stalline structure of the 

 iron-rich forms I have already referred to. The external form as 

 they come to earth varies almost indefinitely, as shown in plates 2, 4, 

 6, and 7. From the almost wholly metallic types there is a some- 

 what abrupt gradation through the stony irons shown in the sec- 

 tion of the pallasite from Mount Vernon, Kentucky (pi. 3), to 

 the stony forms in which iron may be almost wholly lacking. In 

 the Mount Vernon pallasite the only essential mineral aside from the 

 metallic compounds is olivine, which appears as rounded blebs rarely, 

 if ever, with crystal faces, held in a mesh or sponge of iron. Between 

 the iron and the olivine there is commonly a thin plate of schreiber- 

 site. This iron contains a considerable quantity of lawrencite, which 

 exudes from a freshly cut surface as ferruginous drops of moisture, 

 resulting in some instances in serious disintegrations. Occasional 

 examples of these stony irons have been found in which the olivines 

 are so thoroughly shattered as to constitute a breccia with a metallic 

 cement. Such a one is that from Admire, Kansas, a slab of which is 

 shown in plate 5. It is clearly shown here that the metallic portions 

 are of secondary origin and have been introduced subsequent to the 

 shattering of the olivine. In figure 3 of plate 5 is given a more highly 

 magnified view of one of the interstitial metallic areas of the same 

 meteorite. The area is but some three-quarters of an inch in actual 

 length. The dark, outer portion is mainly olivine, the bright border 

 the nickel-iron, and the dark interior a spongy mass of iron with 

 troilite and lawrencite. The acicular forms extending across the dark 

 area are also of iron. Between the bright border and dark interior 

 is a thin belt of schreibersite, which, however, does not show in the 

 figure. 



The nearest approximation in structure among terrestrial rocks to 

 the pallasites is that of the iron-bearing basalts, an example of which 

 from the Nugsauk Peninsula, Greenland, is shown in figure 2, plate 4, 

 in which the white portions are of metal and the darker ground of 

 silicates. The resemblances, on the whole, are quite remote, however. 



Passing to the stony meteorites, as those of the Allegan, Michigan, 

 type (fig. 3, pi. 7), we meet with a class of phenomena which are of 

 greater interest to the average petrographer. According to their in- 

 ternal structure and the presence or absence of feldspar as essential 

 constituents, the stones mostly fall into two general groups — the 

 basaltic and chondritic. The first mentioned are made up of lath- 

 shaped plagioclase with augite and olivine or enstatite, as the case 



