Art. VII. I Watson, Virgilina Copper District. 107 



stituent is shown and more or less of the original rock texture 

 preserved. While this is true for the feldspar constituent, the 

 original bisilicate constituent is completely altered in every slide 

 studied, without any indication as to what the mineral origin- 

 ally was. Considering the age and composition of the rocks, 

 it seems remarkable that any of the original minerals or struc- 

 tures should be preserved at present. When shown, the tex- 

 ture varies from a partial microophitic to microlitic in the non- 

 phorphyritic types, with the same variation and composition of 

 the groundmass in the porphyritic rocks denoted. 



The constituents present are plagioclase, bluish to light 

 green amphibole, chlorite, epidote, zoisite, calcite, iron oxide 

 (partly magnetite), quartz, and apatite. Of these only the feld- 

 spar, a part of the iron oxide (magnetite), and apatite are orig- 

 inal. Both chlorite and epidote, intimately associated with 

 more or less hornblende, are abundantly developed in most of 

 the sectioTiS, sometimes one, sometimes the other predomina- 

 ting ; but the two are at all times intimately connected. 



In the porphyritic and non-porphyritic types the feldspar 

 is present as lath-shaped crystals, showing the broad twinning 

 lamellae of the albite type. Twinning after the Carlsbad law 

 was observed in several instances. In the ground mass of the 

 porphyritic rocks and in the fine-textured non-phophyritic types 

 the feldspars are microlitic, with the boundaries less sharp and 

 well defined than for the lath-shaped feldspars, and the twin- 

 ning is not at all or only slightly indicated. Sometimes the 

 feldspar grouping is suggestive of the sheaf-like arrangement 

 described by Clements^ in similar volcanic rocks of the Hemlock 

 formation of Lake Superior. Poikilitic texture is well devel- 

 oped in many of the larger feldspar laths. 



Feldspar is the only porphyritically developed mineral, and 

 it consists of fairly large, stout laths of broadly striated plagio- 

 clase, with maximum extinction angles measured on the twin- 

 ning planes of 14 to 20 degrees, which would apparently indi- 

 cate an acid plagioclase, probably near oligoclase. 



' Monograph No. xxxvi, U. S. Geol. Survey, 1899, p. 99. 



