18 Field Columbian Museum — Geology, Vol. i. 



prevail upon the earth, in the absence of air or free oxygen and of 

 water. The lack of the first is indicated by the phosphide of iron, 

 schreibersite, which would, in the presence of oxygen, have been 

 changed to a phosphate ; also by the fact that the iron and nickel are 

 in the elementary condition and not oxidized, as they are upon the 

 earth's surface. The absence of water is proved by the fact that no 

 hydrous minerals are present in meteorites. 



It is known that either the atmosphere in which the meteorites 

 were formed or one through which they at some time passed, con- 

 tained a large amount of hydrogen, from the fact that it can be ex- 

 tracted in large quantities from some of the metallic meteorites. It 

 was, too, under a much higher pressure than is that of the earth's 

 atmosphere, since Graham obtained from the Lenarto meteorite 2.85 

 times its volume of mixed gases, of which hydrogen formed 85%. 

 Under the pressure of the earth's atmosphere it is difficult to make iron 

 absorb more than its own volume of this gas. The reducing action 

 of this hydrogen-laden atmosphere must be very great and in gen- 

 eral it may be said that meteorites differ from analogous terrestrial 

 rocks in containing in a reduced state substances which occur as 

 oxides upon the earth. 



Considered as mineral aggregates, meteorites may be conveniently 

 divided into three classes according as they are made up chiefly of 

 iron, partly of iron and partly of stone or chiefly of stone. 



The meteorites of the first class have been called by Maskelyne 

 aerosiderites (from dr t p, air, and oifypov, iron) or by Daubree holosider- 

 ites, (8A"?, whole, ottypo?, iron). The term is frequently shortened to 

 siderite but the abbreviation is objectionable on account of the liabili- 

 ■ ty of its confusion with the mineral of the same name. The meteorites 

 of the second class are called by the same authors derosiderolites (dijp, 

 air, oid-qpos, iron, and W<>$, stone) or syssiderites (o&v, with, otiypos, iron). 

 Those of the third class are known as aerolites (typ, air and ).{0o?, stone,) 

 or by Daubree are divided into the two groups of sporado-siderites 

 (o7:opd$ t scattered, oidypo?, iron,) and asiderites (d without, oidijpog, iron.) 



The aerosiderites, as is indicated by their name, are made up 

 chiefly of iron. This is, however, always alloyed with nickel. The 

 percentage of iron in the mass varies between 87% and 97% and that 

 of nickel from a fraction of one per cent to 15%. 



Two exceptions to this are known among irons supposed to be 

 meteoric. One is that of Octibbeha Co., Miss., which contains 38% 

 of iron to 60% of nickel and the other that of Santa Catarina, Brazil 

 (97-108) which bears 64%of iron to 34%of nickel. It is possible, how- 

 ever, that the latter is of terrestrial origin. 



