CLARKE AND STEIGER: METALLfC ELEMENTS 61 



0.00326 per cent of Pb, and 0.00029 of Zn. Many other deter- 

 minations of the hea\'y metals in rocks are scattered through 

 the literature of geology but these examples are sufficient to illus- 

 trate what has long been known. The researches of Forchham- 

 mer, of Sandberger, and of Dieulafait are familiar to geologists, 

 but they lack the quantitative basis which is supplied by the com- 

 posite analyses given here.^ The heavy metals are widely dif- 

 fused throughout the crust of the earth, and generally in determi- 

 nable proportions. The order of abundance, as now ascertained, 

 appears to be Ni, Cu, Zn, Pb, As, with, of course, local variations. 



With the aid of the estimate here given for zinc, which is near 

 0.005 per cent of ZnO or 0.004 Zn, it becomes possible to gain some 

 notion as to the relative abundance of cadmium; for the two metals 

 are commonly associated. In 10906 shipments of zinc ores from 

 Webb City and Joplin, Missouri, mostly in carload lots, Waring^ 

 found an average percentage of 57.96 Zn and 0.358 Cd, The 

 ratio is 1 Cd to 162 Zn. From 42 analyses of sphalerite given 

 in Hintze's Handbuch der Mineralogie, the mean ratio is 1 to 163. 

 From 82 analyses of European zinc ores, cited by Jensch,^ the 

 ratio 1 to 277 appears. The mean of these three estimates is 1 

 to 201 ; that is, in round numbers, zinc seems to be about 200 

 times as abundant as cadmium. A more precise estimate can 

 hardly be made at present; but the figure is better than no esti- 

 mate at all. It has a quantitative basis, and is therefore some- 

 thing more than a mere guess. If the percentage of zinc in the 

 earth's cinist is 0.004, then that of cadmium is of the order of 

 0.00002. 



In the course of the regular rock analyses made in the laboratory 

 of the Geological Survey, many determinations have been made of 

 elements of minor quantitative importance. These determi- 

 nations are numerous enough to fix their numerical significance 

 between maximum and minimum limits as follows : 



* For literature references see Survey Bulletin 491, The Data of Geochemistry, 

 pp. 600-602, 643. * 



* Cited by Siebenthal in U. S. Geological Survey, Mineral Resources, 1908, 

 1, 796. See also, for other data, Waring's paper in Journ. Amer. Chem. Soc, 26, 

 16. 



^ Ahren's Sammlung chem. techn. Vortriige, 3, 201. 



