EARTH'S CRUST WASHINGTON". 297 



arrangement in both bodies. It is also quite in harmony with the 

 general idea of arrangement according to specific gravity or " grav- 

 itative adjustment." 



We may conclude therefore that the metallogenic elements are rare 

 on the earth's surface and do not show intense spectrum lines in the 

 sun, because they are too deeply buried in both. Connected with 

 this, however, is the difference in the chemical relations already 

 pointed out, the significance of which is as yet problematical. 



It might be pointed out here that such a theory of the vertical 

 distribution of the elements seems to be opposed to Chamberlin's 

 hypothesis of the planetesimal origin of the earth, though the matter 

 can not be discussed in this paper. Attention may only be called to 

 the fact, probably very significant in this connection, that the melt- 

 ing points of the oxides and silicates, the typical natural compounds 

 of the petrogenic elements, are much higher than those of the sul- 

 phides and arsenides, the typical natural compounds of the metal- 

 logenic elements. The bearing of this will be discussed elsewhere. 



Much more might be said of this suggestion of the distribution 

 of the elements of highest atomic weight and greatest density at 

 the center. The idea is not wholly new, having been held specu- 

 latively by others. One might even recall, to pass from science to 

 fiction, that the idea was, in a way, foreshadowed by Jules Verne, 

 who in one of his stories describes a comet or huge meteorite com- 

 posed of telluride of gold. 



CORRELATION OF THE ELEMENTS. 



But we have wandered far from our proper topic, the crust of the 

 earth, having reached not only the center of the earth, but the sun, 

 and become enmeshed in somewhat transcendental chemical specula- 

 tion. Let us come back to the surface of the earth. 



Before returning, however, to the consideration of the actual crust 

 and its rocks, it may be as well to examine briefly a feature of the 

 mutual relations of the elements (for the most part petrogenic), that 

 is shown us by chemical study of the rocks and of the many minerals 

 with which we are acquainted. Since the chemical analysis of rocks 

 and minerals began to assume large proportions, so that sufficient 

 and sufficiently accurate data became available, it has been noticed 

 that certain elements are prone to be found in rocks of certain gen- 

 eral compositions, and also in association with one another in min- 

 erals. In other words, there has been observed a certain correlated 

 distribution of the elements in the earth's crust — that is, in the rocks 

 and minerals composing it — by which certain of the elements tend 

 to occur together in greatest abundance or most often, while other 

 elements are seldom if ever found along with these. As this is a 



