294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



they may probably be placed with these. The metals of the ruthenium 

 and the platinum groups clearly belong here, because of their occur- 

 rence as metals, and because of the existence of the sulphide of ruthe- 

 nium and the arsenide of platinum as the only native compounds 

 known. 



On referring to the periodic classification presented in Table III, 

 it will be seen that the intermediate, meandered zone, where the 

 petrogenic and the metallogenic elements interlock, shows a very 

 large proportion of elements with atomic weights that are quite far 

 removed from whole numbers, which would imply, as has been sug- 

 gested by Harkins, that this is especially the region of isotopes. 

 Whether this is fortuitous or whether it is (if it be true) connected 

 with the division here suggested of the elements into the petrogenic 

 and the metallogenic groups, is quite unknown, and it is needless 

 here to speculate upon the subject. 



It should be mentioned that the relations between the positive and 

 the negative elements, and their occurrence in nature as minerals, 

 as set forth above, form an elaboration and an extension of what 

 Clarke has already called attention to, 20 namely, " In combination 

 unlike elements seek each other, and yet there appears to be a pref- 

 erence for neighbors rather than for substances that are more re- 

 mote. * * * The elements of high atomic weight appear to seek 

 one another, a tendency which is indicated in many directions, even 

 though it can not be stated in the form of a precise law. The gen- 

 eral rule is evident, but its significance is not so clear." A possible 

 significance, or rather a possible connection between this rule and 

 the occurrence of the elements, both as to their relative abundance 

 and their mutual relations, in the earth's crust and below it, may be 

 suggested here, as a somewhat speculative hypothesis. 



THE INTERIOR OF THE EARTH. 



The hypothesis (already adverted to), that the interior of the 

 earth is composed, at least in part, of an iron-nickel alloy like 

 that which composes many meteorites, is commonly held. This 

 is based on the mean density of the earth, its rigidity and magnetic 

 character, and the composition of many meteorites, the siderolites, 

 which may be regarded as fragments of a preexisting large body. 

 Following Charles Darwin and Durocher, who published their 

 view in the first half of the last century, the idea is now held by 

 many that the material composing the interior of the earth is 

 arranged, in a general way, according to relative density, 21 there 



20 Clarke, F. W., " The Data of Geochemistry," U. S. Geol. Survey, Bull. 330, p. 35, 

 1908 ; and Bull. 695, p. 39, 1920. 



21 See, for example, Suess, The Face of the Earth (English translation), vol. IV, p. 547, 

 1909 ; Daly, Igneous Rocks and Their Origin, pp. 162-168, 1914. 



