SOME EXAMPLES OF ROCK VARIA TION 



379 



of the finest grained forms possess a very good parallel texture. 

 Others are noticeably porphyritic. A few have a poikilitic tex- 

 ture. Less common is an approach to the ophitic texture of the 

 dolerites. Most commonly of all, however, the rocks are gran- 

 itic in texture. The color variation is not great, and is chiefly 

 in dark brown or greenish-black tones. The important mineral 

 constituents are feldspar, biotite, hornblende, pyroxene, and 

 olivine. Apatite, sphene, zircon, rutile, octahedrite, brookite ( ?) , 

 and iron oxide occur as accessory minerals. White and brown 

 mica, chlorite, hornblende, talc, serpentine, sphene, rutile, and 

 calcite occur as secondary minerals. 



Plagioclase and orthoclase-feldspar are both present. The 

 last is, however, of doubtful occurrence. The plagioclase is 

 labradorite, and shows its usual characters. 



Hornblende is the most striking component in the majority 

 of the sections. It is present in three different varieties, all of 

 which occur in anhedra. The most prominent kind is a red- 

 dish-brown hornblende, which has a dark green hornblende 

 commonly associated with it, and frequently in zonal intergrowth 

 with it. This hornblende occurs without the green kind, but 

 the green is invariably associated with the brown variety. The 

 two are optically continuous in the intergrowth. It is possible, 

 though not susceptible of proof, in the sections examined, that 

 the green hornblende is the incipient alteration of the brown 

 hornblende. The pleochroism is strong in the following colors: 



Brown hornblende. 



a b 



Light yellow or red, Reddish-brown, 

 with tinge of green. 



Green hornblende. 



a 



Greenish-yellow. 



Yellowish or brown- 

 ish-green. 



Same as b, or else a darker 

 reddish-brown. Excep- 

 tionally it is a light yel- 

 lowish-brown. 



c> b»a 



Darker olive-green. Fre- 

 quently with bluish tinge. 



c>b>a 



