160 CARNEGIE INSTITUTION OF WASHINGTON. 



earth, which were brought together and classified in Professional 

 Paper 99 of the Geological Survey (1917). In its main features this 

 resembles other averages made previously, but as it is based on the 

 largest number of good analyses yet available it is the most authorita- 

 tive. Included in it are estimates of the percentages of several of the 

 rarer elements, such as zirconium, barium, strontium, chromium, 

 vanadium, and nickel. 



It appears that 12 elements (oxygen, silicon, aluminum, iron, cal- 

 cium, sodium, potassium, magnesium, titanium, phosphorus, hydrogen, 

 and manganese) constitute about 99.6 per cent of the earth's crust, 

 leaving only about 0.4 per cent for all the other elements (about 80 in 

 number) . It is noteworthy that (with the exception of iron, aluminum, 

 nickel, and manganese) the common and "e very-day" metals neces- 

 sary for our civilization (such as copper, silver, gold, zinc, tin, lead, 

 mercury, platinum, antimony, and bismuth) are either not present in 

 igneous rocks or are present in scarcely detectable amounts. 



Study of the chemical relations of the elements found in minerals 

 and rocks shows that they are divisible into two groups. The ''petro- 

 genic" group comprises all of the most abundant elements, which 

 compose the igneous rocks, the oceanic waters, and the atmosphere, 

 together with some rarer ones. These petrogenic elements occur typi- 

 cally as oxides, simple silicates, alumino-silicates and other complex 

 silicates, aluminates, fluorides, chlorides, and sulphides. They do not 

 normally form mineral arsenides, antimonides, selenides, tellurides, 

 bromides, or iodides, nor do most of them occur in nature uncombined. 

 The "metallogenic" elements are not found in igneous rocks (except 

 rarely in traces), but they occur as ores. Most of them occur fre- 

 quently as native metals, and their typical mineral compounds are sul- 

 phides, selenides, tellurides, arsenides, antimonides, sulpharsenites and 

 other sulpho-salts, bromides, and iodides. They do not occur normally 

 as primary oxides, silicates, aluminates, fluorides, or chlorides. With 

 the metallogenic elements are included the metals (copper, silver, gold, 

 etc.) mentioned above. 



The distinction is well brought out in the periodic classification of 

 the elements, in which the two groups are sharply separated and occupy 

 the two opposite sides of the table, with the "triad" elements (such 

 as iron, cobalt, and nickel) transitional between them. It is probable 

 that the sharp separation of the two groups is significant of important 

 differences in the relations of the elements to the structure and the 

 evolution of the earth. There is reason to believe that, while the outer 

 part of the earth (the "crust") is composed almost wholly of petro- 

 genic elements, the central core is probably composed of metallogenic 

 elements, with an intermediate zone of iron-nickel-cobalt, probably 

 associated with borides, carbides, phosphides, and sulphides. A 

 similar distribution is believed to obtain in the sun, and there may 



