290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



be a very definite and distinctive difference between these two groups 

 as regards their general chemical relations — a difference that has 

 apparently not heretofore been observed. 



Intermediate between the upper and lower part of the scheme is a 

 zone, series 5 to 8, including elements that our study of minerals and 

 rocks shows to belong partly to the rock elements, and partly to the 

 ore elements. It is found that their relations to the one or the other 

 are clearly distinguished by tracing a meander that separates them 

 into alternate or interlocking vertical columns, the spaces thus made 

 opening above into the division of the rock elements and below into 

 that of the ore elements. Thus, as we shall see, Eb and Cs, Sr and 

 Ba, Yt and La, Zr and Ce, Cb?, and Mo are to be considered as rock 

 elements; while Cu and Ag, Zn and Cd, Ga and In, Ge and Sn, As 

 and Sb, S, Se, and Te, and Br and I, are ore elements. These differ- 

 ences are indicated by some of their general chemical relations 

 and by the facts of their occurrence as minerals — that is, as com- 

 ponents of the earth's crust. 



Flanking the main part of the table is group 0, the column of the 

 inert gases, from helium to niton. At the bottom are the radio- 

 active elements, chiefly radium, thorium, and uranium, with others 

 (some more or less hypothetical) that have been recently discovered. 

 On the right is the column of group 8, that of the triads. Of these, 

 iron, cobalt, and , nickel are to be considered as rock elements, and 

 the two triads of the platinum metals as ore elements. 



It may be as well to suggest here, and to use henceforward, two 

 terms as a matter of convenience. We may call the " rock elements " 

 petrogenic and the "ore elements" metallo genie. These terms are 

 not only short and self-explanatory but, having an adjectival 

 form, are convenient for use. The distinctive chemical differences 

 between the petrogenic and the metallogenic groups of elements, 

 as regards their occurrence in the earth's crust, will be now set 

 forth; and, it may be said, these differences seem to divide them 

 into two "natural" groups, which may be of significance in a 

 study of the constitution of the earth. In the present paper it is 

 best not to go very deeply into technical mineralogical details, and 

 only the main facts will be stated, leaving the details for presenta- 

 tion elsewhere. 



The petrogenic elements occur normally in nature as primary min- 

 erals, forming oxides, silicates, fluorides, and chlorides; but never, 

 or only exceptionally, as sulphides, selenides, tellurides, arsenides, 

 antimonides, bromides, or iodides. With the exceptions of iron and 

 nickel they are never found in the form of native metals. The metal- 

 logenic elements, on the other hand, normally (as primary minerals) 

 form sulphides, selenides, tellurides, arsenides, antimonides, bro- 



