VIRGINIA LOCALITIES 489 



several specimens crystal faces. The magnetite contains on analysis 

 approximately 1 per cent of TiOg. 



The inclosing rock varies from even granular to porphyritic in textnre 

 and is the quartz-bearing variety of hypersthene syenite, the dominant 

 rock type of the igneous complex forming the central core of the Blue 

 Eidge Mountains in Yirginia.'^^ Farther down the northwest slope of the 

 mountain, near the Shepherd home, excellent exposures of schist occur, 

 followed by granite. 



Fresh allanite. — The larger inner ^Dortion of each mass and fragment 

 of the mineral collected seemed entirely fresh and showed the following 

 physical properties : Color, black ; streak, gray ; cleavage, indistinct ; frac- 

 ture, uneven to subconchoidal ; luster, vitreous; and hardness, about 6. 

 Some surfaces are broken by tiny veinlets of a gray but unknown sub- 

 stance, which become more emphasized in the weathered crust of the 

 mineral. With this exception, the fracture surfaces of the mineral appear 

 very homogeneous. When examined under the microscope, the mineral 

 is found to be composed of two forms, one of which is sensibly isotropic, 

 the other birefracting and derived from the isotropic type by alteration 

 or inversion, probably by alteration. The optical constants are given on 

 page 481. An analysis of the fresh mineral is given in column I of the 

 table below. 



The allanite from the adjoining county on the northeast (Bedford), 

 an analysis of which is given in column VIII of table on page 479, is 

 grayish black and is appreciably lighter in color than the Eoanoke County 

 mineral. It weathers light yellow and buff to reddish brown. No analy- 

 sis has been made of the weathered portion of the mineral from the Bed- 

 ford County locality. 



Weathered crust of allanite. — Every piece of the mineral, both large 

 and small, taken from the opening on the Shepherd farm was coated with 

 the decomposition product of allanite derived by weathering. This 

 weathered crust ranged up to nearly a quarter of an inch in thickness in 

 some of the larger pieces, and some of the smaller masses were entirely 

 decomposed throughout without any trace of the original mineral. The 

 color of the crust varied from yellowish brown to deep red brown, the 

 latter being dominant, with frequently an outer exceedingly thin layer 

 of a light gray to yellowish buff. The crust has a decidedly deeper red 

 color in powdered form. 



It is very fine-textured but porous, appears uniform in general appear- 

 ance, and readily crumbles between the fingers to a powder which is usu- 



" Watson and Cline : Bull. (Jeol. Soc. Am., vol. 27, 1916, pp. 193-234. 



