342 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 14 



has a single instance been known where the pyrite bodies were intruded 

 by igneous rocks. 



The object of this paper is (1) to place on record observations 

 of the only known case of a pyrite body in one of the principal pyrite 

 mines of Virginia that is cut by a dike of igneous rock, and (2) to give a 

 petrographic description of the rock, since it is unlike the usual types 

 of basic igneous rocks found in the Virginia Piedmont province, and 

 for the added reason that the mine is closed and is no longer accessible. 



OCCURRENCE 



The lamprophyre dike^ described in this paper cuts the pyrite body 

 in the Boyd Smith mine which is located about 2 miles north of Mineral, 

 Louisa County, Virginia. The mine is one of a group of pyrite mines 

 located near Mineral that have yielded large tonnages of ore. Accord- 

 ing to Mr. Neustaedter, the dike was encountered in mining pyrite 

 near the north heading of the so-called "west vein" on the first level 

 about 100 feet below the surface. The ore body at this level is about 

 8 feet wide, strikes approximately N. 25° E., and dips to the southeast 

 about 60°. The dike does not exceed one foot in thickness, cuts the 

 pyrite body in an east-west direction, and dips about 85° south. The 

 same dike was also observed cutting the "east vein" of pyrite. There 

 is no evidence of the dike on the surface. 



PETROGRAPHY 



Megascopic characters. — The general appearance of the rock is 

 that of a typical porphyritic basalt. It is massive and of dark blue- 

 gray color, with small phenocrysts of biotite and nearly black pyroxene 

 in roughly equal amount set in a dense aphanitic groundmass. Polished 

 surfaces of the rock are uniformly darker (nearly black) in color, 

 with groundmass and phenocrysts rather strongly contrasted. 



The dark minerals, biotite and pyroxene, are the only ones rec- 

 ognized in the hand specimen. Biotite, in small glistening crystals 

 up to 2 mm. in diameter, but usually less than 1 mm., is seemingly 

 more abundant than pyroxene in the hand specimen, but not in the 

 thin section. Pyroxene, nearly black in color, is developed in lath- 

 like prismatic crystals up to 5 mm. long and 1>2 mm. thick, and in 



2 Credit for the discovery of the dike belongs to Mr. A. Neustaedter, former mining 

 engineer in charge, to whom the writer is indebted both for a statement of facts relat- 

 ing to its occurrence and for a hand specimen of the rock. The discovery of the dike in 

 1917 by Mr. Neustaedter was communicated to the writer and a hand specimen sent, 

 but pressing duties prevented the earlier preparation of a statement of the interesting 

 occurrence. 



