312 WATSON AND CLINE IGNEOUS DIKES IN VIRGINIA 



times resemble in outline the stout prismatic forms of pyroxene. One 

 of the pseudomorphs removed from the rock was found to be strongly 

 magnetic, and when treated with cold dilute hydrochloric acid effervesced 

 freely, clearly indicating its composition to be magnetite and calcite, 

 although the calcite was masked by the greater abundance of magnetite. 



Microscopic character. — When examined microscopically in thin-sec- 

 tions, the essential minerals of the rock are found to be plagioclase feld- 

 spar (labradorite), orthoclase, quartz, and pyroxene (augite). The rock 

 appears fresh in the hand specimen, but thin-sections under the micro- 

 scope indicate considerable alteration, which is also shown in the high 

 percentages of C0 2 and H 2 in the chemical analysis below. 



The dominant feldspar is labradorite developed in large formless grains 

 and in small lathlike crystals, occupying areas between the larger grains 

 of the rock constituents. A second plagioclase highly sodic and exhibit- 

 ing multiple twinning is present in minor quantity and is identified as 

 albite. Orthoclase is a constant constituent, but is less abundant than 

 labradorite. It occurs in scattered irregular shaped grains, some of which 

 rival in size labradorite. The feldspars are completely altered in some 

 cases to an aggregate chiefly of minute granular calcite, quartz, and 

 kaolin. 



Quartz is an important constituent of the gabbro, but is much less 

 abundant than feldspar. It is developed in anhedral forms of good size 

 interlocked with each other or with feldspar. Liquid and gas inclusions 

 are numerous and frequently needle-like forms of rutile and amphibole 

 are noted, the latter (amphibole) being also observed as inclusions in the 

 feldspar. A peculiar feature of the quartz is its biaxial character, the 

 optic angle being larger than that ordinarily observed in biotite. 



Pyroxene, probably originally present as the varieties augite and hy- 

 persthene, is partially or completely altered chiefly to calcite and magne- 

 tite, with admixed chlorite and quartz. At present only the monoclinic 

 pyroxene, nearly colorless augite, is distinguishable, the alteration prod- 

 ucts of which are calcite, chlorite, and quartz, with only a little magnetite. 

 The dark, nearly black pseudomorphs observed in the hand specimens 

 and described megascopically above were very probably derived from an 

 iron-rich pyroxene of different variety from the existing one — nearly 

 colorless augite — probably a rhombic pyroxene of Irypersthene composi- 

 tion. In thin-sections the pseudomorphs are seen to be composed of a 

 matrix of calcite thickly set with minute crystals of black magnetite, 

 frequently so abundant as to almost entirely mask the substance of the 

 calcite. They exhibit as a rule irregular boundaries, but in some cases a 

 strong tendency toward prismatic forms is indicated. Fractures in the 

 unaltered augite are preserved in the pseudomorphs, but magnetite is 



