46 BULLETIN 149, UNITED STATES NATIONAL MUSEUM 



but the most recent.^ If such have fallen during earlier periods, they 

 must have been a quite different type, or what is more probable, - 

 become so thoroughly decomposed or otherwise altered as to be 

 unrecognizable. 



Owing to the presence of abundant oxygen in our atmosphere, the 

 iron in the terrestrial rocks is nearly always in an oxidized condition; 

 its presence as metal is exceptional. 



Other minerals noted as found in meteorites in smaller quantities 

 and lacking in terrestrial forms are cohenite,^ lawrencite, merrillite, 

 osbornite, and schreibersite, elsewhere described. It will be seen, 

 therefore, that the chief chemical and mineralogical differences lie in 

 the presence of unoxidized combinations in the meteorites, and not 

 in elemental composition. Rarely occur forms of crystallization like 

 that shown in Figure 2, Plate 29, which are more nearly allied to the 

 basalts and pyroxenites. The nearest terrestrial approach to this 

 meteorite are the iron-bearing basalts of Greenland and Ober Cassel, 

 in Germany, which shows native metal dispersed throughout a ground 

 of silicate minerals.^ 



The nearest terrestrial equivalent to the stony meteorites as a 

 whole is a comparatively insignificant group of intrusive igneous rocks 

 to which the name 'peridotite is given. These, like the meteorites, are 

 composed essentially of the minerals olivine and pyroxene, and so 

 close is their analogy that an element found in one may be predicated 

 for the other, though not necessarily in the same form of combination. 

 The most pronounced difference is in the commonly fragmental nature 

 of the meteorite and presence of iron in the metallic state. We know 

 of no volcanic (tuffaceous) equivalent of our peridotites unless the 

 diamond-bearing breccias of Arkansas and South Africa be so con- 

 sidered. A singularly striking similarity lies in the presence of 

 diamonds and platinum in certain members of both groups, though 

 in meteorites only in minute quantities. 



In the following table is given in I the average results of the analy- 

 ses of 63 stony meteorites; in II that of 8 peridotites, the group of 

 terrestrial rocks most nearly allied to meteorites; and in III the 

 average composition of the rocks of all kinds composing the earth's 

 crust.* 



1 This fact was noted by Olbers nearly 90 years ago. Ward's statement as to the Pliocene age of 

 the Lujan mesosiderite seems contradicted by its having been found in "an undisturbed Quaternary 

 formation." 



« Cohenite has been reported by O. Sjostrom in the native iron of Greenland. 



• Chemical analyses show this iron to contain only about 1.90 per cent of nickel. In both cases the metal 

 is secondary and in the case of Ober Cassel derived by reduction from the sulphide pyirhotite. ■' te>i 



< Clarke, F. W., Data of Geochemistry. Bull. 491, U S. Geol. Survey, 1911, p. 27. The 63 analyses of 

 stony meteorites include all varieties of which complete and satisfactory analyses were available. 



