STRUCTURE OF METEORITES MERRILL. 177 



sory, including under the first term those constituting any essential 

 part and the presence or absence of which affects them fundamentally ; 

 while the accessory minerals include those occurring in smaller and 

 usually inconsequential quantities. The essential minerals then arc : 

 Nickel-iron, olivine, orthorhombic and monoclinic pyroxenes, plagio- 

 clase feldspar, maskelynite, and iron sulphides. The accessories are: 

 Carbon, either amorphous or as graphite and the diamond; chrom- 

 ite, cohenite, daubreelite; the gases carbon monoxide and dioxide, 

 hydrogen and nitrogen ; lawrencite, magnetite, oldhamite, osbornite, 

 schreibersite, a calcium phosphate to which the name of merrillite 

 has been given, and tridymite. In addition there is occasionally a 

 small amount of undifferentiated glass. Concerning these minerals 

 a few explanatory remarks seem necessary, since several of the com- 

 pounds are little or quite unknown among terrestrial rocks. 



The metallic iron of meteorites invariably carries nickel and cobalt 

 in amounts varying from 4 to 20 per cent of the former and 0.5 to 2 

 per cent of the latter. The nearest approach to this composition in 

 terrestrial irons is found in the awaruite of New Zealand, which con- 

 tains 67.63 per cent of nickel and 31.02 per cent of iron, and joseph- 

 inite of Oregon which carries theoretically 72.12 per cent of nickel 

 and 27.58 per cent of iron. The Ovifak, Greenland, iron, a constituent 

 of basalt, carries at the maximum only between and 7 per cent 

 nickel. Perhaps the most interesting feature on the part of most 

 meteoric irons is an apparent tendency to separate on crystallizing 

 into alloys of more or less definite composition which owing to their 

 varying solubility give rise to well-defined and characteristic mark- 

 ings known as Widmanstatten figures when a polished surface is 

 treated with dilute acid. These alloys were studied by Reichenbach' 

 in 1861, who gave to them the name balkenseisen or kamacite, hand- 

 eisen or taenite and filUeisen or plessite, the last named being prob- 

 ably a mixture of the other two. The following anatyses of kamacite 

 and taenite from the iron of Welland, Canada, were made by Prof. 

 Davison, of Rochester : 



Constituents. 



Kamacite. 



Iron 



Nickel . . 

 Cobalt.. 

 Carbon . 



100.05 



Taenite. 



99.93 



It should be stated, however, that analyses made by various 

 workers are found not to agree at all closely, a fact doubtless due in 

 large part to the difficulty of separating them perfectly one from 

 another. The etched slice of the Casas Grandes iron on plate 2, fig. 2 



