REPLACEMENT ORE DEPOSITS IN THE SIERRAS 393 



be plainly seen at the App mine at Quartz Mountain in Tuolumne 

 county. He considers this conversion readily explained when 

 it is considered that serpentine is easily decomposed by carbon- 

 ated waters into magnesite and chalcedonic quartz. In this 

 case, as with the fissure veins, the essential feature is the intro- 

 duction of the carbonated waters, and in confirmation of this 

 may be cited the composition of this peculiar alteration product 

 as given later in the table (Analysis No. 1 508) . The atomic com- 

 position of this rock is quite similar to the atomic composition of 

 serpentine with the addition of carbon dioxide ; while the 

 difference in mineral composition is very striking, the original 

 serpentine being a silicate and the alteration products carbonates. 

 Such a deposit cannot be called a vein and it is likewise from 

 the above standpoint not a replacement deposit, for the original 

 elements are largely still there but in new combinations. There 

 are some facts which will now be presented which at first glance 

 suggest that the quartz, carbonate and mariposite deposits above 

 described have not originated from the alteration of serpentine 

 in place, but have resulted from the metasomatic alteration of 

 dikes rich in soda, and hence may be called replacement 

 deposits. 



Lying just east of Moccasin Creek 1 in Tuolumne count)', is 

 a white dike which extends from the mouth of the creek in a 

 southeasterly direction. The larger portion of the dike lies 

 east of the creek and crosses the road to Priest's about 0.6 kilo- 

 meters east of the bridge over Moccasin Creek. This dike has 

 been rather fully described in a previous publication. 2 It is com- 

 posed largely of soda-feldspar or albite, with quartz and musco- 

 vite locally abundant. A green aegerite-like mineral, and radial 

 tufts of bluish amphibole are likewise present at some points. 

 Throughout the greater part of its course the dike is bordered 

 by serpentine on the west and greenstone on the east. 



At numerous points this dike has been exploited for gold. 

 Some of it is plainly mineralized, containing specks of iron 



1 See Sonora folio of the Geological Atlas of the U. S. 



2 Seventeenth Ann. Rep. U. S. Geol. Surv., Part I, p. 664. 



