LINDGREN.] COMPOSITION OF ALTERED GRANITE. 643 



verted into a milky opaque mass, composed of a great number of 

 small crystals. The also abundant apatite is not attacked at all by 

 any metasoinatic process. The idiomorphic pyrite is chiefly contained 

 in the altered feldspars, but also included in the quartz. When the 

 pyrite crystals are larger the quartz in the vicinity often shows very 

 strong undulous extinction, but some small perfect crystals also lie 

 in the fresh, unchanged quartz, usually attached at one side to a 

 bunch of sericite fibers. An assay of this rock gave 0.05 ounce of 

 gold and 0.5 ounce of silver to the ton. 



Analysis II shows the composition of the altered granite. The 

 chemical change is on the whole slight. Silica shows a small increase, 

 alumina a decrease ; a very slight decrease is also noted in the iron 

 oxides, lime, magnesia, and titanic acid, while the whole of the soda 

 and the baryta has been carried away, and the percentage of potash 

 has been considerably augmented. Sulphur and carbon dioxide have 

 been introduced, and the quantity of chemically combined water 

 has been increased. 



With the aid of the data furnished by the microscopic investiga- 

 tion, the analysis may be calculated as follows. The lime has been 

 calculated as carbonate, excepting that necessary for apatite and 

 titanite, the whole of the magnesia likewise as carbonate, and as 

 much of the ferrous oxide as the remaining carbon dioxide would 

 permit. The remainder is sericite and quartz. The amount of free 

 quartz has been estimated according to the quantity of oxides 

 available for sericite. 



S 95 



Fe._ .83 



Pyrite 1.78 



P 2 O, 17 



CaO.- .22 



Magnesium carbonate 

 FeO 



CO, .. 



Ferrous carbonate 1.45 



Quartz 42.00 



