Surface of Meteorites. 129 



rapidly as it is formed, the heat must quickly begin to pene- 

 trate towards the interior of the body, even where its ingre- 

 dients offer so inferior a conducting-material as spherules of 

 magnesian or ferro-magnesian silicates united in different 

 degrees of aggregation, and, indeed, often forming a very in- 

 compact structure. The explanation I have heretofore offered of 

 the " pitting " of the surface of a meteorite was, that the want 

 of homogeneity of the mass permitted the heat to penetrate 

 more rapidly from its exterior in some places than in others, 

 so that the sudden expansion due to the almost instantaneous 

 accession of enormous temperature on the surface of the stone 

 tore out small pieces of it and flung them off from the swiftly 

 moving mass, the interior of which must be presumed to be 

 the more brittle from the meteoric body entering our atmo- 

 sphere with the temperature of the " cold of space " (see Phil. 

 Mag. January 1863). Even were one large outer shell from 

 this cause to break away from an inner core, we should expect 

 the surfaces to present projections and depressions correspond- 

 ing in part to the denser or less dense, the better or the less 

 well conducting parts of the mass, and partly also of course to 

 any directions of weaker cohesion in the stone arising from 

 other causes. 



The greater fusibility of some than of other of the mineral 

 ingredients of the meteorite (for instance, of the more ferrous 

 as compared with the less ferrous and more magnesiferous sili- 

 cates) may in certain cases explain some of the peculiar pitlike 

 indentations, as, for instance, those in which a rather deep 

 irregularly shaped hole is met with, perhaps half an inch in 

 depth ; and in the fusion and reincrustation of a freshly broken 

 surface, after explosion and fracture, this cause possibly helps 

 in increasing the variolar irregularities of the surface. Another 

 cause of these irregularities suggests itself in the differences of 

 combustibility of the ingredients of a meteorite ; for, in a strict 

 sense of the word, in most meteorites the bulk of the ingredi- 

 ents — all in fact except the purely magnesian or calcio-mag- 

 nesian iron-free silicates — are combustible, i. e. can undergo 

 further oxidation ; for this is true, be it remembered, not 

 merely of the iron and the troilite (FeS), but also of the mag- 

 nesio-ferrous silicates bronzite and olivine. But I cannot believe 

 that this is an active agent in the pitting of the surface, how- 

 ever it may contribute to the brilliancy of the fiery meteor. 

 For it is no uncommon thing to find enclosed in the black 

 oxidized glaze of a meteorite little pepites of nickel-iron un- 

 oxidized ; and in the Busti meteorite the yellowish glaze 

 covered alike the augite and the Oldhamite (CaS), without 

 offering any indication of the latter having been burned awav 



Phil. Mag. S. 5. Vol. 2. No. 9. Aug. 1876. K 



