2')2 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



They are about a centimeter in diameter, in well defined six-sided i)riLsms, 

 and are of a dull greenish brown color. Owing to the toughness of the 

 surrounding material the crystals so far obtained are badly fractured 

 and without good terminations. 



Apatite — This mineral, which until recently had been found only in 

 microscopic crystals, has lately been discovered by Mr. L. H. Merrill in 

 masses of considerable size embedded in white quartz from a vein in the 

 shaft at Roclv Creek. The largest found were some 2 centimeters in 

 diameter, of distinct hexagonal outline, and of a light yellowish-green 

 color. The majority of the specimens are, however, distorted and of 

 irregular form. On account of their being embedded in tough, massive 

 quartz it is impossible to obtain perfect crystals, and all yet obtained 

 are more or less shattered and broken. 



Black tourmaline. — This mineral is a very common constituent 

 of the District rocks, occurring usually as small acicular crystals pene- 

 trating the quartz of veins. It has been found also in radiating forms 

 on amphibollte, incurved trigonal prisms 4 to 5 millimeters in diameter 

 embedded in prochlorite, as a very fine granular aggregate in quartz 

 veins, and more rarely in columnar crystals of considerable size in the 

 quartzose portions of the amphibolite. It has also been found by Pro- 

 fessor Robinson at the Howard University shaft penetrating calcite. 



Menaccanite. — Aside from the usual grains scattered throughout 

 the rock this mineral has been taken from the Foundry Branch tunnel 

 in thin plates interlaminated with quartz or more commonly prochlorite. 

 The thickness of the plates varies up to 2 millimeters, usually being 

 about half this amount. They are commonly curved and an inch or 

 more in diameter. The associated minerals are prochlorite, epidote, and 

 quartz. 



Sphene. — This was first found in imperfect and broken crystals of 

 a yellowish-green color, quite clear, and semi-transparent. Owing to 

 its high fusibility and its evident pronounced cleavages in two direc- 

 tions there was at first some doubt as to its exact nature. Particles of 

 the clear portions were therefore picked out and submitted to Professor 

 Clarke for analysis, with the results given below : 



Ignition 54 



SiOj 30.10 



TiOi 40.82 



CaO 28.08 



Mg. O 40 



Mn. O Trace. 



99.94 



Specific gravity 3, 452 



Later more and better specimens were found in which the character- 

 istic wedge-shape crystalline form is unmistakable. The largest speci- 

 men obtained is about 2 centimeters in greatest diameter and occurs, 



