60 



CLARKE AND STEIGER : METALLIC ELEMENTS 



D. Composite of 329 igneous rocks, all American. Determi- 

 nations on 90 gram portions. 



In the red' clay a trace of molybdenum was also detected by Dr. 

 Hillebrand. 



TABLE 2 

 Summary op Data from Composite Analyses 



These figures give quite clearly the order of magnitude of the 

 several percentages, and they are corroborated by the work of 

 other investigators. In a series of 36 igneous and metamorphic 

 rocks of British Guiana, Harrison found ^ mean percentage of 

 0.025 copper. In 23 of his samples lead was also sought for, and 

 detected in 5 of them, the maximum amount being 0.02 per cent. 

 In a typical specimen of the Columbia River basalt Wells found 

 0.034 of copper, and the same quantity was determined by Jensen 

 in an andesite from Fiji. In the porphyries of Leadville, Colorado, 

 Hillebrand was able to determine lead. Out of 18 samples, taken 

 at points remote from ore bodies, the average amount found was 

 0.002 per cent of PbO. One porphyry also yielded 0.008 per 

 cent of zinc oxide, and a rhyolite contained 0.0043 per cent. 



In four rocks granite, porphyry and diabase from the Archean 

 of Missouri, Robertson determined the following percentages of 

 lead, zinc, and copper: 



Pb 0.00197 to 0.0068; average, 0.004 



Zn .00139 to .0176; average, .009 



Cu 0.00240 to 0,0104; average, 0.006 



The adjacent limestones also carried these metals, but in slightly 

 smaller proportions. Similar results were obtained by Finlayson 

 from igneous rocks adjacent to lead mines in Great Britain. His 

 averages are Pb, 0.0032 per cent, and Zn, 0.028. In the lime- 

 stones and dolomites of the Dubuque region, Iowa, Weems de- 

 termined lead and zinc. The average of nine samples gave 



