PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 67 



ON PRO CHLORITE FROM THE DISTRICT OP COLUMBIA. 

 By OEOROE P. miERKILI^. 



The presence iu tlie schists of the District of Columbia of a chloritic 

 miueral not apparentl3^ resulting from the alteration of the included 

 hornblende or mica was recognized by the writer a year or more ago, 

 but until lately no sufficient amount of the material was found for an 

 exact determination of its mineralogical species. While searching for 

 rutile on Foundry Run, some IJ miles northwest of the city of Wash- 

 ington, not long since, he was, however, fortunate enough to find a mass 

 of this mineral of sufficient size and purity to admit of satisfactory ex- 

 amination. 



The mineral occurs in the form of a compact aggregate of small scales 

 of a beautiful deep green color, the individual laminne of which are from 

 one to two millimeters iu diameter and of very irregular outline. 



Examined by the microscope it is found to be biaxial, though the 

 axial divergence is not large. Sections across the cleavage show a fan- 

 shaped or radial arrangement of the laminae, which are of a yellowish- 

 green color, V)ecomiug green whenever the cleavage liues correspond 

 with the i)lane of vibration of the light. The polarization colors are dull 

 and of a greenish-gray hue. In hardness it is about 1.5, and its specific 

 gravity, as determined by a Jolly balance, is 2.835. Before the blow- 

 pipe it fuses with difficulty on the thin edges, and becomes slightly 

 magnetic. Chemical analysis by Prof. F. W. Clark, chief chemist of 

 the Geological Survey, resulted as follows : 



Per cent. 



Silica 25. 45 



Alumina 17. 88 



Magnesia 15. 04 



Iron iirotoxide 24. 98 



Soda 67 



•.Vater 14. 43 



98.45 

 The associated minerals are very dark green hornblende, a yellowish- 

 gray finely granular crystalline miueral which is believed to be zoisite, 

 much iron pyrites, and, in the more quartzose portions of the rock, 

 black tourmaline and rutile. 



National Museum, April 10, 1884. 



