1885.] PEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 351 



NOTES ON THE MINERALOGY AND LITHOLOGY OF THE DISTRICT 



OF COLUMBIA. 



By OEOKOE: p. lUERRir.!.. 



Since the piiblicatiou by the author of two previous papers bearing 

 on this subject,* the work on the extension of the Washington City 

 water- works has been commenced and rapidly pushed toward comple- 

 tion. This work includes the construction of a tunnel through the un- 

 derlying rock in the northwestern portion of the city, extending from 

 the distributing reservoir beyond Georgetown to the new reservoir now 

 in process of construction near Howard University, a total distance of 

 21,400 feet. To facilitate the construction of this tunnel, shafts have 

 been sunk to the proper deptbs at Howard University, Champlain 

 avenue, Kock Creek, Foundry Branch, and at the western extremity 

 near the present distributing reservoir northwest of Georgetown. 



The materials taken from these shafts and the tunnel have been care- 

 fully examined, and while, with perhaps a single exception, no rocks have 

 as yet appeared of a different kind from those known to occur on the 

 surface, several minerals have been brought to light never before re- 

 ported from this vicinity, and which, in the majority of cases, have not 

 yet been found in other than very small and perhaps microscopic forms 

 in the surftice rock. It is the object of this paper to call attention to 

 those minerals of special interest that, so far as the author is aware, 

 have not been reported heretofore.t 



Epibote. — As a constituent of the rock epidosite this mineral occurs 

 in very minute crystals in a vein of unknown width which crosses the 

 New Cut road a few rods east of Foundry Branch. The rock is very 

 fine grained and of a yellowish- green color, and is frequently associated 

 with massive quartz, prochlorite, and menaccanite. A similar vein has 

 been passed through by the tunnel at Chamj^lain avenue. In curved 

 columnar forms the epidote sometimes occurs embedded in the prochlo- 

 rite. We have received from Professor Eobiuson a specimen of this 

 form in which the crystals are some 10 centimeters long and 5 to 6 milli- 

 meter in breadth. They are broken several times transversely, and the 

 fractures have become filled with prochlorite. The finest crystals yet 

 seen were found by Mr. L. H. Merrill embedded in the quartz veins of 

 amphibolite which had been thrown out from the tunnel at Eock Creek. 



"Preliminary notes on the Crystalline Schists of the District of Columbia, these 

 Proceedinijs, vol. vi, 1883, pp. 159-161, and on Prochlorite from the District of Co- 

 lumbia, these Proceedings, vol. vii, 1884, p. 67. 



t It may be well to state here that Professor Eobiuson, of Howard Uuiversity, has 

 kindly volunteered to collect for the Museum a full suit of specimens at intervals of 

 every 50 feet throughout the entire length of the tuunel. We hope to make a thor- 

 ough study of these after the completion of the work. 



