308 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



that state occurs as irregularly segregated patches in a foliated or 

 gneissoid epidotic granite, a very different rock, at least macroscopi- 

 cally, from the country rock at Milams Gap, and that, so far as he 

 is aware, it is confined to Madison county, North Carolina, and 

 Cocke and Sevier counties, Tennessee. 



Geology and Petrography 



The Blue Ridge at Milams Gap, Virginia, is a single range, com- 

 posed entirely of igneous rocks. Proceeding from Stanley, on the 

 Norfolk and Western Railroad, by the public road which runs south- 

 eastward through the village of Marksville, the transported material 

 of Hawksbill creek is finally left behind in large measure and the 

 igneous rock in place is first encountered at an elevation of 1,260 

 feet.^ 



The rock, the hypersthene-akerite of the succeeding pages, con- 

 tinues in almost unbroken continuity to the olivine-basalt, mapped as 

 beginning at an elevation of 2,600 feet. The first fragments of 

 unakite were observed at 1,640 feet, but are probably not in place. 

 The first distinct mass of the material occurs at from 80 to 125 feet 

 below the sharp turn in the road represented on the 2,000-foot con- 

 tour. Much epidosite is associated with it. No sharp lines of 

 demarcation between it and the akerite can be distinguished, and the 

 mass cannot be more than. 10 feet wide. The mass, which is prob- 

 ably a continuation of that noted higher up on the road, trends north, 

 75° to 80° east. It is represented on the map as being rather regular, 

 or as a dike. The masses seen, however, are too irregular to be 

 classed as true dikes ; they may more properly be termed irregular 

 patches. The main mass of unakite occurs on the eastern slope of 

 the ridge, where it is associated with akerite, a large mass of which 

 is found in the basalt on this side of the ridge. Most of the material 

 here is in the form of debris. 



It is to this spot that collectors should go for the material in 

 quantity, " the best specimens being obtained along a foot-path that 

 cuts ofif the first curve in the road in descending on the east side."^ 

 I may add that excellent specimens may also be obtained in th'e 

 formerly cultivated fields lying on the right as one continues down 

 the road, just beyond the head of the path referred to. 



In order to make the discussion of the origin of the unakite more 

 intelligible, the results of the microscopic examination of the country 

 rock will first be presented. 



^ All elevations were determined by means of an aneroid. 

 2 Fontaine's letter. 



