LOCALITIES OF HYPERSTHENE-DIABASE. 341 



dikes of " greenstone," composed of hornblende and white feldspar. It was 

 in a specimen from Culpeper county, Virginia, near the southern end of this 

 Triassic area, that we first noticed the presence of hypersthene. 



Localities of Occurrence. — A few localities where the rocks to be described 

 below occur in their typical form may be mentioned : 



About three miles north of Rapidan, a station on the Virginia Midland 

 railroad, are to be seen two rounded knolls of similar shape and size, as 

 rather conspicuous objects in the landscape. They are sometimes called the 

 " Twins," although several other names are given to them by the people in 

 the neighborhood. On the top of the knob nearer' to the railroad, beautiful 

 perpendicular, pentagonal and hexagonal basaltic columns form the face of a 

 cliff. Some of these columns are sixty feet high and from twenty to twenty- 

 five feet in diameter. A ridge runs around from one knoll to the other, 

 making a curve like a horseshoe. The cliffs face the center of the curve. 

 The stone from the side of the "Twins" is quarried under the name of 

 granite. It is in reality hypersthene-diabase. Along the railroad between 

 Rapidan and Mitchell stations for a mile or more the same rock appears in 

 the cuts. 



A few miles east of Culpeper Court-House, Mount Pony stands out prom- 

 inently as an isolated peak, commanding such an extensive view from its 

 summit that it has been used as a signal station upon various occasions. 

 The greater part of this peak is composed of a rock of the same character, 

 although the hypersthene is less conspicuous in it than in the other speci- 

 mens. 



Not far east of Brandy station, on the same railroad, a hummock called 

 the " Dumpling " is hid away in the woods. This hillock is composed of the 

 same rock. Other knobs and dikes occur in this region. 



Similar rocks occur in New Jersey and Pennsylvania, as we have learned 

 through the courtesy of Mr. N. H. Darton, who kindly put into our hands 

 for examination the collection of New Jersey trap rocks made by him for 

 the United State Geological Survey. In the thin section number 152, from 

 " three miles south of Milford, New Jersey, in Pennsylvania," we find hypers- 

 thene to be abundant. The rock seems to be identical with that from the 

 " Twins." Section number 294 shows the same rock as occurring at Point 

 Pleasant, New Jersey. With reference to the latter rock Mr. Darton says * 

 that it is very similar to the typical palisade trap. Several other of these 

 sections show the probable presence of an orthorhombic pyroxene. 



These localities will suflSce to show that the hypersthene-diabase is not 

 merely a local variety. 



At only one locality have we thus far noticed the olivine-hypersthene-dia- 

 base. A dike of considerable width occurs crossing the Virginia Midland 



* Bulletin U. S. Geological Survey, no. 67, 1890, p. 68. 



