10 



BULLETIN 94, UNITED STATES NATIONAL MUSEUM. 



In the Casas Grandes, Toluca, and many other irons, these plates 

 are arranged parallel with the faces of an octahedron, as shown in 

 the examples in the introductory series. Such are known as octa- 

 hedral irons. Other irons yielding no Widmanstatten figures give, 

 on etching, li»es which the mineralogist Neumann showed might re- 

 sult from a twinning of a cube about an octahedral face. These are 

 known as hexahedral irons, an example of which is shown in the slice 

 from Scottsville, Kentucky (No. 77) , or in the large " Couch " iron. 

 Still other irons have no regular structure, sometimes, indeed, being 

 almost uniformly homogeneous. Such are classed as ataxites, an 

 example of which is shown in the specimen from Deep Springs, North 

 Carolina (No. 470). Cohenite is the name proposed by Weinschenk 

 for an iron carbide of a tin-white color, found first in the meteorite 

 of Magura and subsequently in other irons. 



Oldhamite. — This name was given by Story-Maskelyne, in 1862, 

 to a calcium sulphide found by him in the meteorite of Busti, and de- 

 scribed in detail in the Philosophical Transactions of the Royal 

 Society of London for 1870. The mineral is of a pale, chestnut- 

 brown color when pure, though often covered on the outer surface by 

 a gypseous oxidation product. It occurs in the form of rounded 

 granules, with cleavages essentially rectangidar, imbedded in the 

 pyroxenic constituents. Between crossed nicols it is isotropic, and is 

 considered to belong undoubtedly to the cubic, or isometric sys- 

 tem. Its specific gravity was found to be 2.58. Boiled in water it 

 was decomposed, yielding a bright yellow solution of calcium poly- 

 sulphide and an insoluble residue. 



Olivine. — A magnesium and iron silicate of the fornuila (MgFe) 

 SiOi ; relative proportions of magnesia and iron are, however, some- 

 what variable, as shown in the following analyses : 



The mineral rarely occurs in good crystal form except in the 

 porphyritic chondrules. It is of all meteoric minerals perhaps the 

 most abundant and widespread, sometimes, as in that of Warrenton, 

 Missouri, composing a very large proportion (75 per cent) of the mass 

 of the stone. It is rarely, if ever, wholly absent, even the iron meteor- 

 ites showing in most cases included granules. It is a common and 

 widespread constituent of terrestrial igneolis rocks. 



