GEOLOGY OF THE HAYSTACK STOCK 213 



VARIETY D. MONZANOSE: QUARTZ- BEARING ORTHOCLASE-GABBRO 



Toward the interior of the stock the proportion of ferromagnesian 

 minerals increases and the rock becomes more coarsely crystalline. 

 The specimen described is a grayish-brown gabbro occurring about 

 hahway betweeen Mud and Blue Lakes, and is composed chiefly of 

 feldspar, pyroxene, and biotite. 



Under the microscope the following minerals are seen to be present 

 and are mentioned in the order of abimdance : plagioclase, orthoclase, 

 augite, hypersthene, magnetite, biotite, quartz, serpentine, and 

 apatite. 



Plagioclase occurs in nearly idiomorphic crystals. Carlsbad 

 twins about 1™°^ long are common, and some crystals show zonal 

 structure. Both andesine and labradorite are present. Minute 

 dust-like opaque bodies and larger ones of brown glass, with a little 

 apatite, pyroxene, and magnetite, are included in the plagioclase. 

 Orthoclase and quartz, micrographically intergrown, fill the inter- 

 stices between other minerals. Augite is more abundant than 

 hypersthene. 



Biotite occurs as plates up to 3"^"^ in length, partly altered to 

 hydro-biotite. An analysis of biotite separated from D is given 

 below. Magnetite, highly titaniferous, is included in the other 



ANALYSIS OF BIOTITE FROM VARIETY D. GEO. STEIGER, ANALYST 



SiOa 33-07 NaaO o . 28 



Al,03. 13.00 K2O 6. II 



Fe^Oa 17.22* H2O- 5.41 



FeO notdet.t H^O-f ". 11. 61 



MgO 11.33 . 



CaO 2 . 45 Total 100 . 48 



*Contams ferrous iron P^Os and TiOj. tDetermined as FcaOj. 



minerals, and is especially abundant in the ferromagnesian con- 

 stituents. A little serpentinized olivine is present. Long prisms 

 of apatite show a tendency to parallelism. The order of crystallization 

 was as follows: (i) Apatite and magnetite; (2) Plagioclase and the 

 ferromagnesian silicates; (3) orthoclase and quartz. Feldspar is 

 slightly kaolinized, hypersthene is partly altered to bastite, biotite to 

 hydro-biotite, olivine to serpentine. An analysis of D from a speci- 

 men taken midway between Mud and Blue Lakes (see Fig. 3) is as 

 follows : 



