PRESIDENTIAL ADDRESS- -SECTION 1!. 3 I 



space, that the constitution of the meteoritic parent body might 

 show us to some extent the constitution also of the earth. We 

 cannot, of course, say that the two would be identical in com- 

 position, Ijut, at the same time, the strong analogy between 

 meteoritic and terrestrial materials cannot be denied. 



Thus, Suess, arguing from the chemical characters of 

 meteorites and their relation to those of terrestrial ultra-basic 

 rocks, has suggested that the earth consists of three principal 

 zones — 



1. A metallic barysphere. rich in iron and nickel. 



2. An intermediate zone, rich in magnesia and silica ; and 



3. An outer crust, rich in silica and alumina. 



Holmes supposes that there is a terrestrial zone correspond- 

 ing in order of density to each of the ])rincipal types of 

 meteorite. Daly, in his recent work on the igneous rocks and 

 their origin, also advocates a coarse stratification or zonal 

 arrangement of the materials of the earth according to their 

 density. 



With regard to the radio-active characters of meteorites, we 

 have so far very scanty information. The results, however, 

 of Holmes' analyses show an absence of radium in the iron- 

 meteorites, which, in view of the analogy between meteoritic 

 and terrestrial material, strongly supports the conclusion of the 

 absence of radium and, therefore, of uranium from the metallic 

 core of the earth. The stoney meteorites showed, on the whole, 

 a radium content rather less than that of terrestrial ultra-basic 

 rocks, while a considerably lower amount appears in the iron- 

 stone class, the radio-active matter occurring in verv minute 

 quantity in the silicate minerals. In the meteorite parent body, 

 then, evidentlv, the radium content decreased with depth until 

 it died out altogether, and showed, to some extent, a similar 

 general type of distribution to that which we have seen 

 apparentlv obtains in the earth. The inference is that radio- 

 activitv, and, therefore, the amount of radio-active materials 

 tends to increase towards the sin^face of such bodies, and would 

 show a jjroeressive decrease with increase of depth, and of 

 density of the constituent materials. 



On the whole, the evidence from meteorites certainly lends 

 support to the conclusion that the radio-active elements in the 

 earth are concentrated towards the upper part (^f the crust. The 

 concentration on the earth, however, is of a more advanced 

 order, having been probably to a great extent controlled by 

 selective mineralogical differentiation assisted by aqueous and 

 atmospheric agencies, which we have seen were apparently 

 absent on the meteoritic parent body, so that it may be supijosed 

 to have proceeded f^ari f^assu with the evolution of the more acid 

 or siliceous rocks. 



The problem of geological time is another of the cjuestions 

 which is being reconsidered in the light of radio-activity. In 



