GEOLOGY OF THE HAYSTACK STOCK 207 



built crystals are present. Its composition is that of oligoclase-andesine, 

 Ab3Ani. 



Orthoclase and quartz are relatively abundant, together nearly 

 equalling plagioclase. Orthoclase, frequently microperthite, occurs 

 as anhedra with a maximum length of i™'", and is also graphically 

 intergrown with quartz. It also occurs as zones surrounding crystals 

 of plagioclase. Quartz occurs as anhedra from o.i™"^ to 0.5"^"^ 

 long. 



Hornblende forms irregular anhedra, usually about 0.4™"^ long, 

 which inclose a large number of minute bodies of magnetite and 

 biotite. A part of the hornblende, from its fibrous form, is believed 

 to be secondary, possibly to pyroxene, though there is nothing in its 

 outline which suggests the form of pyroxene and it may have been 

 derived from an original hornblende. Biotite occurs as anhedra 

 smaller than those of hornblende. Magnetite is included in the 

 ferromagnesian minerals, and to a less extent in plagioclase. 

 Long prisms of apatite are present. The order of crystallization 

 was as follows: (i) Magnetite and apatite; (2) hornblende and 

 biotite; (3) plagioclase; (4) orthoclase and quartz. The feldspars 

 are slightly altered to kaolin and biotite to a lighter colored mica and 

 to chlorite. An analysis of type A from the west spur of Baboon 

 Mountain (See Fig. 3 p. 222) is given below: 



ANALYSIS OF VARIETY A. GEO. STEIGER, ANALYST 



Si.O 65 



AI2O3 14 



Fe^Oj 2 



FeO I 



MgO 2 



CaO 3 



Na,0 3 



K.0 3 



06 H.O- 31 



71 H2O+ 1. 10 



82 TiO. 61 



31 P^Os 18 



48 MnO 18 



43 BaO 10 



86 SrO..... 05 



Total 99-68 



NORM AND MODE OF VARIETY A 



The norm of A calculated from the analysis is given in column 

 I of the table below. The mode is given in column II . According to 

 the quantitative classification this rock is adamellose. ' Its chemical 

 composition closely resembles that of a quartz-mica-diorite, occurring 

 in a stock at Crandall Basin, Hurricane Ridge, Yellowstone National 



