1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 35.' 



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Vol. Till, ]\o. 33. l¥ashin§^ton, D. €. Sept. ^, 1 8 85. 



as do most of the others, embedded in prochlorite. They are all very 

 brittle and none have been removed entire from the matrix. The pro- 

 nonnced cleavage property above alluded to I am now inclined to think 

 to be caused by repeated twinning, as recently described by Dr. G. H. 

 Williams.* 



Stilbite. — This mineral is of common occurrence forming a thin 

 coating on the joint surfaces of both the hornblendic and micaceous 

 rocks of the District. No really good material was found, however, 

 until the work on the tunnel was commenced. The most common form 

 is that of thin radiating blades coating the surface of the rock. Mr. 

 True, of the National Museum, has, however, found it in small rectan- 

 gular prisms implanted upright on thin layers of calcite. These forms 

 are usually but a few millimeters in length and of a faint yellowish 

 color. The largest specimens yet obtained were taken by myself from 

 a narrow vein of calcite in the amphibolite, about 1,250 feet west from 

 the eastern end of the tunnel by Howard University. These are some 

 15 millimeters in diameter and about the same length. 



Laumontite. — This, in connection with stilbite and prochlorite, is 

 now one of the most common minerals of the District, although, until 

 identified by myself from material gathered from the water-works tun- 

 nel, its presence has not before been recognized. The common form is 

 that of a mass of imjierfect crystals associated with calcite in narrow 

 veins in the mica or hornblende rock. I have also found it in four- 

 sided prisms with the ordinary oblique terminations, in small geodic 

 ■cavities in the calcite veins. The best crystals are about 1 centimeter 

 long by 4 millimeters broad and of a white or reddish color. 



Heulandite. — This occurs in minute right rhomboidal prisms from 

 1 to 2 millimeters in diameter coating the surfaces of natural joints or 

 in small cavities in the prochlorite. So far as examined the crystals 

 ])i'eseut no unusual forms, and are of a grayish or yellowish-gray color 

 with the characteristic pearly luster. 



Prochlorite.— Attention was first called to the occurrence of this 

 mineral in the District by myself in April, 1884.t Since that time con- 

 siderable quantities of the material have been taken from the water- 

 works tunnel, a few rods west of the shaft at Foundry Branch. It 

 occurs most abundantly associated with the vein of epidote rock above 

 alluded to. Specimens have also been taken from the tunnel near Eock 

 Creek, Champlain avenue, and Howard University, showing it to be 

 generally distributed throughout the rocks of the District. I have also 

 found it on the surface in narrow veins in the amphibolite near the 

 mouth of Foundry Branch, where it empties into the Potomac River. 



July 20, 1885. 



*Aiu. Jour, of Sci., June, 1885, p. 486. 

 t These Proceedings, vol. vii, 1884, p. 67. 

 Proc. Nat. Mus. 85 23 



