SILVER AND GOLD IN ERUPTIVE ROCKS. 581 



what may be assumed to have been in the form of sulphide. This would 

 correspond to 0.002277 per cent, of impure galena, assuming impure galena 

 to bear the relation to PbO of eight to nine. If this galena carries 0.00823 

 per cent. Ag, as above, 0.002277 X 0.00823 = 0.0000177 per cent. Ag, or 

 0.0051 ounce to the ton. would be the amount it contributed to the total 

 silver contents of the rock. If this be added to the amount to be derived 

 from pyrite, 0.0156 + 0.0051 = 0.0207 ounce, it is still less than the average, 

 0.0265 ounce, given for the above average rock. 



But the tests for lead show that a considerable portion is contained in 

 the silicates (in the three specimens in which this test was made, about 

 thi-ee-nfths of the whole amount) ; and, if the silver is assumed to be neces- 

 sarily associated with the lead in the rock, this would amply account for 

 the remaining 0.0058 ounce. 



Of the other porphyries the most significant are the Lincoln and the 

 Gray, which, as has already been shown, are practically the same type of 

 rock. Both have a much larger proportion of basic silicates than the White 

 Porphyry. Of the two the Gray is to outward appearance the more de- 

 composed, but in the Lincoln Porphyry microscopical examination shows 

 that alteration of the basic silicates has already set in, and it is probable 

 that the more decomposed appearance of the Gray Porphyry is due to the 

 action of surface waters on the other constituents, mainly the feldspars. It 

 is noticeable that the Lincoln Porphyry from Clinton gulch, which con- 

 tains the most silver, is the only one which contains pyrite ; also, that the 

 others are from a region where there has been a considerable concentration 

 of metals in ore deposits, which is not the case, so far as known, in Clinton 

 gulch. 



With the exception of the Sacramento Porphyry the other rocks have 

 no apparent association with important ore deposits. It is significent that 

 the diorite given in Table IV contains augite, hornblende, and mica, 

 whereas two other diorites assayed, and in which no silver was found, con- 

 tained a very small proportion of basic silicates. In the recent eruptive 

 rocks there is also an apparent relation between the amount of basic 

 minerals and the contents in silver. In Nevadite, in which they are almost 



