296 Pratt — Corundum in North Carolina. 



rock has been cut across for nearly 100 feet in a direction 

 about at right angles to the strike. 



The fresh rock at the bottom of the shaft is somewhat varied 

 in appearance and while it all does not show any definite gneis- 

 soid structure, the more finely divided portions are distinctly 

 so. There are streaks, a few inches thick in the rock, that are 

 composed almost wholly of a plagioclase feldspar. Some por- 

 tions of the rock are decidedly porphoritic and contain pheno- 

 crysts of a light gray amphibole, a centimeter in diameter, in 

 a groundmass of feldspar. A large part of the rock is made 

 up, however, of small roughly-outlined prismatic crystals of an 

 amphibole, probably hornblende, and irregular fragments of 

 plagioclase feldspar. The hornblende is almost black in color 

 but in thin splinters it has a bronze luster and a deep resinous 

 color. Biotite of a deep brown color occurs sparingly, and a 

 pink garnet is rather abundant. It is this part of the rock that 

 is of a gneissoid structure and in which the corundum occurs. 

 The corundum is of a light to a purplish pink color and in 

 nodules up to two or three centimeters in diameter. There 

 are some streaks in the rock that are very highly garnetiferous, 

 composed essentially of the garnet and plagioclase feldspar, or 

 of the garnet and biotite. Chalcopyrite occurs very sparingly 

 in these portions of the rock. Small particles of graphite have 

 been observed in the coarsely crystallized portions. 



Prof. L. V. Pirsson has kindly made a microscopical exam- 

 ination of this rock, the results of which are embodied in the 

 following paragraphs. 



In thin section the microscope disclosed the minerals, horn- 

 blende, plagioclase feldspar, garnet, biotite, muscovite, stauro- 

 lite and rutile. Hornblende is the most common, forming 

 about two-fifths of the section, while of the remainder, plagio- 

 clase and garnet occur in about equal quantities and the others 

 in comparatively insignificant amount. 



" The hornblende is formless but tends to irregular columns 

 almost invariably extended in the plane of schistosity ; it has 

 very rarely a somewhat stringy tendency in its cleavage but is 

 usually homogeneous in broad plates. Its color is a clear olive- 

 brown and it is somewhat pleochroic but not strongly so. It 

 is everywhere dotted by the small grains of garnet, which rarely 

 show good crystal form. The garnet occurs associated also 

 with the plagioclase." 



" The plagioclase occurs twinned according to the albite law 

 only. In sections perpendicular to 010, the lamellae show 

 extinctions as great as 30° and the plagioclase is therefore rich 

 in lime and as basic as labradorite, which it probably is. It 

 shows strong evidence of shearing movement in the rock : it is 



