136 DESCRIPTIVE GEOLOGY. 



It resembles the rock described by F. H. Bradley/ from the Unaka Range, 

 a part of the Blue Ridge, which forms the boundary between North Caro- 

 lina and Tennessee. East of Bruin Peak, the granite assumes a more uni- 

 form character, composed chiefly of orthoclase and quartz, with beds cany- 

 ing more or less hornblende or mica. Both hornblende and mica-gneisses 

 are found here, the former, an exceptionally beautiful rock, being made up 

 largely of crystals of dark, brilliant hornblende, with an admixture of white 

 feldspar, which is, however, at times almost entirely wanting. Much of the 

 feldspar is shown, under the microscope, to be well-developed plagioclase, 

 while the quartz appears to be a subordinate constituent. 



Hornblende also appears to be present in considerable amount in some 

 varieties of the mica-bearing gneisses. These gneisses become so fine- 

 grained that they may be properly classed as micarslates, consisting of 

 minute grains of quartz, dark- greenish flakes of biotite, and some little 

 white mica, probably muscovite. Under the microscope, thin sections dis- 

 close abundant grains of magnetite. Zirkel calls attention to the interesting 

 manner in which the octahedi-al crystals of magnetite are penetrated by 

 prisms of colorless mica. 



On Mount Zirkel, a somewhat similar, series of mica and hornblende 

 beds occur, which are finely exhibited in the rock exposures of the glacially- 

 eroded canons. Here the bedding is distinctly seen on a large scale, and 

 presents a series of rock formations, highly altered, with an exceedingly 

 variable lithological habit. Hornblende, associated with both orthoclase 

 and plagioclase, forms the prevailing rock, while other beds are made up of 

 mica and orthoclase, carrying some little hornblende and probably always 

 a small amount of triclinic feldspar. They are characterized by a banded 

 structure, in which the individual minerals are separated in layers, but vary- 

 ing greatly in width and outline, expanding from a mere line up to several 

 inches, and again narrowing down to the former dimensions, and mingling 

 in with the other constituents. Usually, the dark hornblende layers will 

 be penetrated by small quantities of feldspar, g.nd the feldspar bands, in 

 turn, finely striped with hornblende. Segregations of the diff'erent minerals, 



'Am. Jour. Sci., May, 1874, 519. 



