earth's crust — Washington. 293 



main groups the several elements there belong, and allude to another 

 feature of interest regarding this part of the table. 



Rubidium and caesium are known only as silicates, caesium form- 

 ing the rare metasilicate pollucite, and both entering in small amount 

 into other silicates, as beryl, lepidolite, and a few others. Strontium 

 and barium, apart from their sulphates and carbonates of secondary 

 origin, enter only into silicates, a barium silicate forming a member 

 of the feldspar group, and both being the bases in some of the 

 hydrous zeolites. The proper position of yttrium and lanthanum, 

 in group 3, is somewhat uncertain, but they both enter into the com- 

 position of various silicate minerals, and are not known as sulphides, 

 arsenides, etc. The position of zirconium and cerium is quite clear; 

 both form silicates, zircon being especially widespread among gran- 

 itic rocks, and they also enter into the composition of some members 

 of the pyroxene group. The position of columbium (niobium) is 

 also somewhat uncertain, as no silicates of it are known, but it may be 

 basic in some titanates, and its general affinities as to mineral occur- 

 rence would place it almost surely with the petrogenic elements. 

 Closely related to it, and occurring with it almost always, is tantalum, 

 whose true place is uncertain. Minerals containing these two ele- 

 ments, however, are very rarely met with. The researches of Hille- 

 brand 19 have shown that molybdenum is very widely distributed 

 among the more silicic igneous rocks, such as granites, so that, even 

 though its most abundant mineral is the sulphide, it should be reck- 

 oned with the petrogenic elements. 



Of the intermediate metallogenic elements, the positions of copper 

 and silver are s unquestionable, as both occur combined most fre- 

 quently as sulphides and other such minerals. Silver does not occur 

 as a silicate or oxide, but silicates and oxides of copper are not un- 

 common, though these are of secondary origin. The same may be 

 said of zinc and cadmium, the oxide and silicate of zinc being sec- 

 ondary. Gallium and indium are found only in zinc sulphide 

 (sphalerite), and germanium occurs only as a sulphide with silver 

 and tin. Though tin is most commonly met with as the oxide (as 

 well as a rare silicate), yet sulphides of it are known, so that, in 

 spite of its frequent occurrence as oxide, it is to be reckoned with 

 the metallogenic elements. Arsenic and antimony, as well as selen- 

 ium and tellurium, belong, of course, in this group, as does sulphur, 

 the necessary inclusion of which among the metallogenic elements 

 carries them somewhat into petrogenic territory, and renders the 

 meander somewhat unsymmetrical toward this end. Bromine and 

 iodine, as we have seen, are only met with in nature in combination 

 with metallogenic elements (except in solution in sea water), so that 



•» Hillebrand, W. F., Amer. Jour. Sci., vi, p. 209, 1898. 



