THE TERRESTRIAL DISTRIBUTION OF RADIUM 33 



The mean densities of the chief groups of meteorites are 

 classified in the following table : 



Type of Meteorite. Mean density. 



^chondrites I g ( 3 '2 J 



Chondrites ) t 3*5 ) 



Siderolites 1 Iron . stone . . . U'8 > 

 Lithosidentes ) <. 6*5 > 



Holosiderites Iron 7*8 



The achondrites do not widely differ in chemical composition 1 



from terrestrial ultra-basic rocks. The chondrites are also very 



similar, except as regards the small proportion of free metal 



which is usually present, and even this can be matched in certain 



terrestrial rocks which carry native iron. The field relations of 



the ultra-basic rocks, as well as their superior density, indicate 



that they are genetically connected with a deep-seated zone 



which underlies the more acid rocks of the earth's crust. On 



the meteoritic analogy, which is based as much on structures as 



on densities, it may be assumed that they extend to the first 



surface of discontinuity, so constituting not only the deeper 



portions of the crust itself, but also the first great zone which 



lies beneath it. The second zone would presumably be formed 



of material like that of the siderolites ; the third would compare 



closely with the lithosiderites, and, finally, corresponding to the 



holosiderites, would come the central metallic core. On this 



distribution the mean density of the earth would be 5 ' 1 . Although 



the earth's actual mean density is 5-53, the discrepancy is not 



without significance, for it is highly probable that under the 



pressures obtaining in the ultra-basic zone minerals would form 



of higher density than those in the stone meteorites. It has 



been suggested by Fermor that the reason why such minerals 



{e.g. garnet) do not occur in meteorites is that a general recrystal- 



lisation would accompany the disruption of, and consequent 



relief of pressure in, the parent body. 



The view that meteorites allow us to read at our leisure 

 many of the secrets which are otherwise locked up in the earth's 

 interior was originally held by Boisse, 1 Meunier, 2 and Daubree, 3 

 who arranged meteorites according to their densities, and founded 

 an analogy solely on that arrangement. The same idea has more 



1 Mem. de la. Soc. des Lets. Sci. et Arts de VAveyron, vol. vii. p. 168. 



1 Cours. de Geol. ComparSe. 



3 Suess, The Face of the Earth (Eng. trans.), vol. iv. p. 543. 



