64 H. L. Preston — Iron Meteorites, etc. 



figures small, many times imperceptible without a leas, and 

 the troilite nodules small in size and few in number. 



In the first case we would infer that the process of solidifica- 

 tion was a very slow one, permitting the crystalline structure 

 to be large, and the concretionary nodules of the monosul- 

 phide, troilite, to be likewise large. 



It is well known that the process of solidification, or crys- 

 tallization of mineral matter, is a purifying one, and that the 

 tendency is for the purest material in the molten mass to 

 become the first crystallized. In the case of siderites, this 

 would, of course, be the kamacite, and the slower the cooling 

 the greater the amount of impure material cast aside, and the 

 larger the crystals; thus giving the monosulphide a chance to 

 segregate during a longer period of time, causing more numer- 

 ous arid larger troilite nodules than in the second instance 

 where solidification has taken place more rapidly, hence giving 

 less time for the purifying process. In the latter case we 

 would expect a smaller number of the troilite nodules, as well 

 as smaller in size. 



It may be asked, why in the coarsely crystalline meteorites 

 should the exterior surfaces be marked by large and deep pit- 

 tings, while on those in which the crystalline structure is 

 scarcely apparent these pittings should be almost entirely 

 absent. The answer is that when the crystalline structure is 

 coarse, the plates of kamacite are long and broad and the bands 

 of tsenite thick. Thus the tendency of separation in the 

 mass would be between the plates of kamacite and the bands 

 of tsenite, and in its passage through the atmosphere fissures 

 by the expansion due to heat, caused by friction, would be 

 made along these planes, and cause them to separate, and thus 

 masses would fall out. The rough ragged edges, where the 

 separation took place, would be further fused and worn away 

 until the base of the cavity was again comparatively smooth, 

 causing large and deep pittings. 



While in the case where the Widmanstatten figures are 

 small, the blades of kamacite are narrow and short, the bands 

 of tsenite thin and there would be a much less chance of sepa- 

 ration along these planes, and the degree of fusion over the 

 whole surface would be comparatively the same, which would 

 produce smooth, rounded exterior surfaces, with but few and 

 shallow pittings. 



In summary the two following points are affirmed : 



First. That siderites have originally been encased in stony 

 matter, and 



Second. That the exterior surface of a siderite will give 

 strong indications as to its internal structure. 



Eochester, N. Y. 



