98 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



yellowish to dirty yellowish-brown color, smooth and extremely thin — 

 a mere skin coating. In several instances there was noted on freshly 

 broken surfaces, small, very thin areas of coal black or smoke-black 

 glass. The cause of this or its relation to the crust is not readily 

 apparent, but it is doubtless the very last fusion product of atmos- 

 pheric resistance before reaching the earth. The usual pittings or 

 thumb marks are present though the rock has become broken into so 

 many pieces that these are not in all cases markedly evident. (PI. 14.) 

 A local slickensided movement is developed along the graphitic areas 

 but which in no case observed extends throughout the mass. It 

 would indicate merely such a movement as would take place within 

 a mass under compression, without the production of faults. On the 

 polished surface scattered particles of metallic iron and iron sulphide 

 are readily observed, but they are extremely irregular in their distribu- 

 tion, and much more abundant in the dark, nearly black enclosures 

 referred to. An interesting feature is the peculiar weathered appear- 

 ance of even a fresh fracture. Fragments broken through the impact 

 of fall and gathered within a few days show dead, lusterless surfaces, 

 as though exposed for many weeks or months. It is probable that 

 this is due to the physical condition of the main constituents, noted 

 later. 



In the thin section the white, chalky mineral referred to is seen to 

 make up the main mass of the rock, though in various conditions 

 of fragmentation from almost perfect forms to mere dust (pis. 16 and 

 17). These are often so crushed, crumpled, and otherwise distorted as 

 to give only undulatory extinctions, and with other optical properties 

 badly obscured. More perfect forms occur as broad plates (pi. 16, fig. 2) 

 with well-defined vertical cleavage lines giving parallel extinctions. 

 Basal sections show imperfect, nearly rectangular prismatic cleavage 

 and the emergence of an optic axis. These facts together with the 

 refractive indices (1.658 -f-) and the results of Mr. Shannon's anal- 

 yses (I, p. 100) leave no question but that the mineral is enstatite. In 

 many sections, however, the mineral shows in polarized light irregular, 

 wavy, and interrupted bandings which extinguish alternately as the 

 stage is revolved, in a manner at first suggestive of the polysynthetic 

 twinning of monoclinic pyroxenes or feldspars. In these cases the 

 broader, more continuous bands give parallel extinctions and show 

 in converged light the emergence of a bisectrix. The narrow, often 

 indistinct and pinched out bands give inclined extinctions running 

 as high as 37°. No distinction in color or refractive indices is notice- 

 able, but there is apparently no question but there is here an inter- 

 growth of orthorhombic and monoclinic forms (ooPx> = ooPoo) in 

 the usual manner. If the analyses made by Mr. Shannon correctly 

 represents this intergrown material (it was selected and analyzed 

 before such an intergrowth was suspected) the proportional amount 



