50 



1st, Basaltic metorites. These are somewhat related to basalt in 

 composition. The typical forms consist of felspar (usually 

 anorthite), pyroxene (augite, diallage, enstatite, &c.), and iron 

 ores. 



2nd, Meteoric peridotites. These are mainly composed of olivine and 

 minerals of the enstatite group. They contain also chromite 

 and picotite. In mineralogical and chemical composition they 

 bear the closest relation to terrestrial peridotites, but they differ 

 in texture. 



3rd, Olivine-iron meteorites. These consist of metallic iron together 

 with minerals found in the preceding group and others, such as 

 schreibersite, troilite and pyrrhotite, which are especially 

 characteristic of the succeeding group. The iron contains 

 varying proportions of nickel and cobalt. 



4th, Metallic meteorites. These consist mainly of iron, alloyed with 

 nickel and cobalt. They contain also schreibersite, troilite, 

 pyrrhotite and graphite. 



The different groups are not separated from each other by hard and fast 

 lines. The first passes into the second by the decrease of the felspathic 

 constituents and the increase of the magnesian silicates ; the second into 

 the third by the gradual introduction of metallic iron ; the third into the 

 fourth by the disappearance of the magnesian silicates. 



If now we arrange the terrestrial and extra-terrestrial rocks in parallel 

 columns, after the manner of DAUBREE, (1) we obtain the following result : 



SP. GK. 



Stratified rocks 2-6 



Granites and felsites ... 2'G5 

 Basalt and Gabbros ... 2-9 

 Peridotites 3'26 



Felspathic meteorites 3 3'5 



Meteoric peridotites ... 3-5 3-8 



Iron-olivine meteorites 5-46 7.29 



Metallic meteorites .. 6-24 7 '95 



Is it possible, as DAUB BEE has suggested, that we may by this means 

 obtain an idea of the composition and structure of the interior of the earth ? 



The connection between terrestrial and extra-terrestrial rocks becomes 

 still closer if we include with the former the metallic nickel-iron (2 > which 

 occurs in large and small masses in certain Greenland basalts. 



Mr. HILL (3) has recently endeavoured to determine the average density 

 of a mass composed of all the known meteorites. The data available are, as 

 he points out, very imperfect, but the conclusion at which he arrives is, 

 nevertheless, very suggestive. It is, " that an assemblage of such meteoric 

 masses as fall upon the earth, if collected from space indiscriminately and 



(1) Geologie experimentale, 1879, p. 545. 



(2) STEENSTBTJP. M. M. Vol. VI., 1884, p. 1. JUDD. Volcanoes, 1881, p. 319. 



(3) On the average density of meteorites compared with that of the earth. G.M.. Decade 

 III., vol. IT., p. 517. 



