18 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



size of meteorites are therefore both largely controlled by the speed 

 of travel. In either case sufficient heat is generated for the fusion 

 and burning away of the immediate surface, the fused material 

 being stripped off nearly as fast as formed, and but a thin coating of 

 material quickly cooled during the last moment of the flight remains, 

 producing the black crust so characteristic of stones or the thin layer 

 of oxide on the irons. The burning does not always take place evenly 

 over the entire surface, the foremost point of the body naturally 

 being most affected, the result being a smooth rounded nose, or 

 "brustseite," from which radiate in all directions furrows or flutings 

 (piezoglyphs) sometimes developed to a remarkable degree of per- 

 fection, as in the stone of Bath Furnace, Ky. (pi. 1). Inequalities 

 in composition, as the presence of troilite nodules or other causes 

 are often productive of deep flutings in the irons, or holes which may 

 extend entirely through the mass, as in the Tucson iron or several 

 of those of Canon Diablo, Ariz. In this connection it should be said 

 that the flattened, sharply edged, and irregular form of the smaller 

 individuals of the Canon Diablo fall are not original, but due to a 

 weathering, which has been productive of the "shale balls" of Bar- 

 ringer and other writers. That this is correct is shown not merely 

 by theu' shape— edges unrounded by fusion — 'but by the occasional 

 finding of a mass with a still unaltered nucleus of metal lying like 

 an oyster in its sheU. Once broken, such usually undergo a further 

 rapid oxidation and fall to pieces. 



The black crust coating the surface of freshly fallen meteoric stones 

 is, as noted, due to the final hasty cooling of the fused material of 

 the meteorite. This is, on the immediate surface, a more or less 

 perfect glass, which is interspersed below the immediate surface with 

 unfused silicate and metallic particles. It is rarely more than a few 

 millimeters thick, as in the case of the Allegan stone shown in Figure 

 1, Plate 1, and Figure 1, Plate 28. Should the stone hold the same 

 relative position in the air for anj^ appreciable distance, the fused 

 material stripped from the nose may accumulate to a greater thick- 

 ness at the rear; it is, however, but a few millimeters thick at best. 



Many meteorites are found traversed by a series of black thread- 

 like veins, which are probably but lines of fracture produced by the 

 shock of impact on entering the atmosphere, as noted later (pi. 29). 



The meteorite differs from terrestrial rocks not more in external 

 characteristics and chemical and mineralogical composition than in 

 the manner in which its various constituents are arranged in relation 

 one to another. While there are certain features, as the presence 

 of metal, that are sufficiently pronounced to enable one at all expe- 

 rienced in such matters to decide almost at a glance upon the meteoric 

 nature of any object, even though not seen to fall, it is nevertheless 

 on a study of the thin sections and surfaces both polished and 



