504 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



chondrules in the light portion are almost wholly very light gray and nearly 

 white, while those in the dark portions are in part of a dark-gray color, 

 although there are white chondrules here also. By reflected light the polished 

 surface shows a structure distinctly brecciated, and in one or two cases it is 

 possible to trace the outlines of a fragment of the darker rock inclosed in the 

 lighter gray, but in the majority of cases this is impossible, and the darker 

 material is so commingled with the lighter that for a long time considerable 

 uncertainty existed in the mind of the writer as to the true nature of the stone. 



^t * if: * * i» * 



The strongest argument in favor of the brecciated nature of the stone seems 

 to lie in the presence of the polysynthetically twinned pyroxenes in the dark- 

 gray chondrules and their absence in the lighter portions. In one instance the 

 line of demarkation between the light and dark portions could be plainly 

 traced in thin section, and the metallic sulphides were found elongated along 

 this line to indicate that it had been an open cleft at the time of their deposi- * 

 tion.' 



It remains to be stated that in the newly cut stone the dark por- 

 tions are plainly not entirely a result of oxidation through weather- 

 ing, as they are distributed regardless of surface contours and are 

 as abundant in the center as about the periphery. By holding the 

 stone so that the polished surface catches the light, lines of demark- 

 ation between the lighter and darker portions can in many instances 

 be very readily made out. But, again, there are places where it 

 would seem most certain that the darker portion is but an oxidized 

 zone about a fragment of the lighter. In short, it does not seem pos- 

 sible to decide the question of brecciation, if one must rely on exami- 

 nation of a polished surface alone. Thin sections were therefore 

 prepared of two portions affording structural differences even to the 

 unaided eye, from which the photomicrographs reproduced in plate 

 79 were made, figure 1 being of the dark portion and figure 2 of the 

 light. It is scarcely necessary to call attention to the marked dif- 

 ference in structure, which is all that was suggested in the first paper. 

 To further illustrate the difference, sections were prepared showing 

 the contact between the two portions, a photomicrograph of one of 

 which with the same degree of enlargement is shown in figure 1, 

 plate 80. 



To assure myself that the apparent greater abundance of chon- 

 drules in the dark portion was not due to their being thrown out in 

 relief by the deposition of interstitial iron oxides, a second like sec- 

 tion was prepared, which, without cover, was then placed section side 

 down over a narrow-mouthed vessel containing a few cubic centi- 

 meters of strong nitric acid, where it was allowed to remain for 36 

 hours, when it was carefully washed and remounted. There was 

 effected an almost complete leaching out of the iron oxide, and of 

 course a portion of the metal and sulphide ; the olivine was also some- 

 what attacked but not enough so to vitiate the wished-for results. 



1 Proc. U. S. Nat. Mus., vol. 52, 1917, p. 422. 



