332 



GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



II is of Gray Porphyry, Quota shaft, Johnson gulch, near Leadville [59a], 

 fresh appearing, but somewhat altered, with the same products as in the former rock. 



The relative amounts of soda and potash indicate an abundant soda-liine feldspar. 

 The titanic oxide found corresponds to the suggestion that the yellow needles in the 

 decomposed biotite are rutile, for the magnetite does not give signs of an intermixture 

 of titanic iron through its alteration products. The presence of strontia in determiua- 

 ble quantities is unusual and worthy of note; it doubtless comes from the plagiochue. 

 Instances of its determination in rocks are rare, 1 though it would probably be found 

 in many cases if sought for. 



Although the large pink or white orthoclase crystals are characteristic of most 

 of the occurrences referred to the Lincoln and Gray Porphyries, still a number of cases 

 were found where the rock seemed identical with these types in every respect, except- 

 ing that the large crystals were wanting. In some bodies of rock, moreover, the large 

 crystals were by no means equally distributed. It seemed therefore desirable to ascer- 

 tain more definitely the source of the alkalies in the rocks analyzed. In each case 

 enough of the large orthoclase crystals had been included in the material used for 

 analysis to give average results. 



In the mass of Mount Lincoln a dike of rock was found which was considered 

 as a representative of the Lincoln Porphyry [78], although it was darker, more com- 

 pact, and contained none of the large pink orthoclase crystals. Alkali determinations 

 gave 2.42 per cent, of potash and 3.15 per cent, of soda, very nearly the same ratio as 

 in the type rock. There was also found 64.10 per cent, of silica. The reduced amounts 

 of all these are doubtless due to the increased quantity of biotite and of ore in this 

 dike rock. 



In the next place the Gray Porphyry, of which the complete analysis had been 

 made, was subjected to further investigation. Alkali determinations were made in the 



1 Streng, Neues Jahrbucli fur Mineralogio, etc., p. 537, 1867. 



