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GEOLOGY OF THE EUREKA DISTRICT. 



Chemical Composition of Granite-porphyry. The original magma injected into 



the limestone through the various openings was, so far as can be told, much 

 the same in its ultimate composition. It is probable that the variation in 

 chemical composition between rocks of different dikes is no greater than 

 that between different parts of the same dike. While structural peculiar- 

 ities across the dike are strongly marked, the mineral constituents remain 

 much the same, although the walls are more acid than the center and carry 

 less ferro-magnesian minerals. As regards tenure of silica the variation 

 between one rock and another would not exceed 5 per cent, with corre- 

 sponding variations in their essential ingredients. It is probable that the 

 silica variations in the bulk of the granite-porphyry would fall within 4 

 per cent. 



The following two analyses of granite-porphyry were made by Mr. 

 Andrew A. Blair: 



Analysis No. 1 may be taken as representing the composition of a 

 large area of the rock west of Wood Cone possessing a granitic structure, 

 the essential minerals having no crystallographic outline. The quartz and 

 feldspar are of medium size and are accompanied by nearly all the accessory 

 minerals recognized in these rocks, including biotite, zircon, titanite, and 

 allanite. 



Analysis No. 2 is made from granite-porphyry obtained from the mid- 



