A NEW OCCURRENCE OF UNAKITE— A PRELIMI- 

 NARY PAPER 



By W. C. PHALEN 



Introduction 



In 1884, Prof. W. M. Fontaine, of the University of Virginia, 

 sent to the U. S. National Museum a specimen of igneous rock of 

 exceptional interest, occurring at Milams Gap (see Fishers Gap, 

 pi. LXix) at the summit of the Blue Ridge, seven miles south of 

 Luray, Va. The rock was entered and placed among the granites 

 under the caption unakite. In the spring of 1903, in response to 

 a letter from Dr. G. P. Merrill, head curator of the Department of 

 Geology, U. S. National Museum, Professor Fontaine sent another 

 consignment of specimens with the associated country rock and at 

 the same time a detailed description of where the unakite might be 

 found. 



Later in the spring the writer spent a day at the locality in the com- 

 pany of Dr. Merrill, and in the fall he spent three days alone 

 in the region, mapping the rocks and making collections as extensive 

 as the limited time allowed. The following brief descriptions are 

 the result of the two excursions. 



Before describing the rocks, attention should be called to plate 

 LXIX. It will be noticed that the boundaries of the fock masses on 

 either side of the road, leading to and from the summit, have been 

 left open. This was rendered necessary as it was not possible to 

 ascertain their true extent in the limited time available; it is 

 hoped, however, that this work may soon be accomplished. Cer- 

 tainly from a petrographical point of view the country is one de- 

 serving of further and detailed study. 



So far as the writer is aware, this Virginia locality is the second in 

 America where unakite occurs in appreciable quantity, the first being 

 the Unaka mountains between North Carolina and Tennessee, 

 whence the rock derives its name. It is highly probable, however, 

 that the rock is not nearly so limited in distribution as this statement 

 would seem to imply. At Marblehead Neck, Mass., for example, 

 some rather imperfect specimens were obtained a few years ago, the 

 unakite occurring as a dike in diorite (see pi. lxx, b). Here the epi- 

 dotic material has undoubtedly resulted from alteration of the colored 



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