SILVER AXD GOLD IN ERUPTIVE ROOKS. 



579 



zinciferous. They are 0.008 and 0.0043 per cent. ZnO, respectively, with 

 an appreciable amount of CoO as well. Of the granites, the one containing 

 lead is of eruptive type, the other rather a granite gneiss. 



Had time and laboratory facilities permitted, these results might have 

 been greatly multiplied and the analyses made much more exhaustive ; but, 

 although the existence of lead in such connection inferentially implies that 

 of gold or silver also, it was thought wiser to obtain direct proof of the 

 occurrence of these metals as well. 



Silver and gold determinations. The quantitative results of these determina- 

 tions, together with a statement of the methods employed, will be found in 

 Appendix B. The following table presents the results given in Table IV 

 in a condensed and more comprehensive form : 



Silver and gold in eruptive rocks. 



In discussing the above results it is important to consider the present 

 condition, position, and composition of the different rock masses. It must 

 be borne in mind also that the negative results are not absolute, but mean 

 merely that the specimen tested does not contain more than 0.0000068 

 per cent, of silver, or 0.002 ounce to the ton. Such quantities are, it is true, 

 almost infinitely small ; so also is the amount of time and water allowed for 

 their leaching almost infinitely great. 



The White Porphyry is the most universally decomposed rock in the 

 Nowhere was it possible to obtain it in an absolutely fresh state, 



region. 



