QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 201 



10 degrees, and the mineral has been identified as uralitic hornblende. 

 The alteration takes place about the margin of the hypersthene and along 

 fracture and cleavage lines. In extreme cases of alteration the change is 

 accompanied by the development of much free iron oxide. The mono- 

 clinic pyroxene present is augite, which frequently exhibits a well devel- 

 oped diallage parting parallel to 100, especially in the more basic facies 

 of the rock. 



Quartz is present in all the thin sections, but the quantity is very 

 variable ; in some it is a very minor accessory, in others it is an important 

 constituent. The principal occurrence of quartz in the rock is as irregu- 

 lar grains of variant size, being the last product of crystallization filling 

 the interstices between the other minerals. It is also developed as rounded 

 inclusions in the feldspar and as graphic intergrowths with both feldspar 

 and hornblende. The quartz-hornblende growths are in all respects like 

 the common micrographic intergrowths of quartz and feldspar. In the 

 five analyses made of the rock from different localities in the Blue Eidge 

 normative quartz is large in amount, ranging from 14.76 per cent to 25.44 

 -per cent. Without exception the quartz of the hypersthene syenites dis- 

 cussed in this paper is some shade of gray in color and does not contain 

 abundant inclusions of rutile needles, as the blue quartz of the quartz- 

 bearing rocks of the Amherst-Nelson counties rutile area and of some 

 granitoid rocks in the northern Blue Eidge region. 



Biotite is sometimes developed in the syenite, both as an original and 

 as a secondary constituent. The pyrogenetic biotite is of the ordinary 

 brown variety, possessing strong absorption and good cleavage, and does 

 not exhibit any unusual characters. In neither occurrence, however, is 

 the mineral usually abundant ; in fact, it was observed only in a few thin 

 sections. At one or two places in the Browns Gap section, Albemarle 

 County, biotite, as an original mineral, is rather plentifully distributed 

 through the syenite, both in hand specimens and in thin sections. 



Hornblende occurs as an original mineral in some thin sections of the 

 syenite and is usually very fresh. Secondary hornblende (uralite) is also 

 formed at times as an alteration product from the hypersthene. The 

 primary hornblende is developed in irregular-shaped masses of fair size 

 and to some extent in small grains. The angle between c and z is nearly 

 20°. Pleochroism is strong: x = pale yellowish brown, y = brown, and 

 z = deep reddish brown. The hornblende frequently exhibits inter- 

 growths with quartz which strongly resemble granophyric structure in 

 feldspar and quartz. In some phases of the rock hornblende-quartz inter- 

 growths are quite common. 



Apatite, magnetite, titanite, zircon, and pyrite were identified in nearly 



