316 WATSON AND CLIXE IGNEOUS DIKES IN VIRGINIA 



these may form an aggregate constituting a psendornorph after the 

 nepheline. 



Hornblende and biotite occur as original minerals in all thin-sections 

 studied. In the south dike hornblende is abundant, with rare flakes of 

 biotite, while in the north dike biotite is the most abundant mafic min- 

 eral, with rare crystals of hornblende. This constitutes the only impor- 

 tant mineralogical difference between the two dikes of nepheline syenite. 

 When hornblende is the chief constituent it occurs in formless grains and 

 crystals with partial boundaries, and in microliths scattered through the 

 colorless minerals or in radiating groups. Hornblende is of green color 

 and pleochroic. x = green, y = light green, and z = greenish yellow. It 

 is associated with brown biotite and green chlorite as alteration products. 

 The inclination of z to c is nearly 10°, and the variety is probably bark- 

 evikite. Hornblende in small grains occurs sparingly in thin-sections of 

 the rock from the north dike. 



Biotite is partly brown and partly green in color, both colors frequently 

 occurring in the same individual, with the green occupying the outer 

 border. In other cases the crystal is entirely brown or entirely green. 

 The green color in some instances is certainly due to chloritization, and 

 this may possibly be the case in most instances where green is shown. A 

 little iron oxide and a black opaque mineral in irregular shaped forms up 

 to 0.5 millimeter in length, probably ilmenite, occur in all thin-sections, 

 but never in abundance. 



Chemical composition and classification in the quantitative system. — 

 An analysis of each of the two dikes of nepheline syenite is given in the 

 table below, the north dike being represented by the analysis in column I 

 and the south dike by the analysis in column II. The two analyses are in 

 very close agreement, and the two rocks, as with nepheline syenite in gen- 

 eral, are characterized by medium low silica, high alumina and alkalies, 

 but especially high soda. 



Chemical Analyses of Nepheline Syenite from Augusta County. Virginia 



North dike South dike 



I la II Ha 



Si0 2 54.77 .912 53.57 .892 



A1 2 3 21.41 .210 22.46 .221 



Fe,0 3 2.46 .016 1.01 .006 



FeO • 0.68 .010 0.92 .013 



MgO 0.18 .004 0.37 .009 



CaO 0.47 .009 1.49 .027 



Na 2 9.53 .153 10.22 .165 



K 2 4.97 .053 5.14 .054 



H 2 0— 0.17 0.43 



H 2 0+ 4.61 .... 2.97 



