GEOLOGY OF THE HAYSTACK STOCK 225 



The mode of the granite is as follows : 



Quartz 25 . 51 Biotite 2 . 78 



Alkali feldspar 60.70 Magnetite 3.50 



Andesine (AbjAria) 4 . 09 • 



Hornblende 3 . 42 Total 100 . 00 



From this mode the following chemical constitution is estimated: 



Si03 70.38 CaO 75 



AI2O3 13-17 Na^O 2 . 10 



FcjOs 2.74 K2O 7.99 



FeO 1.87 



MgO 73 Total 99.73 



From a study of thin sections it is estimated that the alkali 

 feldspar is 15.17 albite and 45.52 orthoclase and that the plagioclase 

 is andesine, Ab3An2. It is assumed that the hornblende is chemically 

 like that from a quartz-monzonite, at Mt. Hoffman, Colo., and 

 that the biotite is chemically like that from a granodiorite at 

 El Capitan, Yosemite Valley, Cal. 



The norm of the granite vein calculated from the estimated com- 

 position is, 



Quartz 24 . 50 Hypersthene 2 . 65 



Orthoclase 47.26 Diopside 68 



Albite 17.82 Magnetite • 3-94 



Anorthite 2 . 78 Total 99 • 67 



According to the quantitative classification the rock is omeose. 

 In composition it closely resembles a granite from Currant Creek 

 Canyon, Pike's Peak, Colo., described by Mr. E. B. Mathews,^ a 

 graphic granite from Omeo, Victoria, described by Mr. A. W. Howitt,^ 

 and a rhyolite from Round Mountain, Rosita Hills, Colo., described by 

 Dr. Whitman Cross. ^ It is not far from the composition of micro - 

 pegmatite estimated by Mr. J. J. H. Teall from a vein-cutting diabase 

 near Kington. 4 



GENESIS or THE ROCKS OF THE STOCK 



The gradational character of the various rocks of the Haystack 

 stock shows that it is geologically a unit and should be classed with 



1 Bulletin No. 148, U. S. Geological Survey. 



2 Transactions of the Royal Society 0} Victoria, Vol. XXIV, p. 1 20. 



3 Proceedings oj the Colorado Scientific Society, Vol. II, p. 33. 



4 British Petrography, p. 401. 



