62 C. M. Wetherill on Itacolumite. 
Percy and others). It occurs in extensive formations in Brazil, | 
the Urals, and in the United States in Georgia, and in North | 
Caroliua, and appears particularly to accompany the diamond. | 
The specimens which I have examined are: zZ 
of slabs of various size from Saraw Mt., Stokes Co., N. C., given © 
to me by Captain Samuel W. Dewey. 2d. Small fragments of 
a large slab in the Museum of the Smithsonian Institution, to 
which it was presented by Charles DeSelding, who obtained it ~ 
in Rio de Janeiro, its locality being Minas Geraes, the celebra- © 
ted diamond region of Brazil. These two specimens have the 
same general appearance; with the exception of color, which 
for the North Carolina rock is a light yellow, and for the Brazi- 
lian mineral a light reddish brown, and of the granular condi- 
tion, which is finer for the North Carolina specimen. ; 
They may be split more readily in one direetion yielding | 
slabs, and there are occasionally small fissures at right angles to ~ 
these layers. There is also in one of my specimens a plane — 
forming an angle of 15° with the plane of readiest cleavage and 
perpendicular to the planes of the fissures. It appearsto bea ~ 
joint and is covered with small crystals of quartz. : 
Scattered through the rock are small scales of mica which — 
are comparatively few in number but of uniform size, thin, and | 
determine the cleavage of the rock. These plates lie in parallel ~ 
planes, as they would settle naturally froma liquid. Where © 
they are abundant in any plane the rock splits there readily. If _ 
a piece of the itacolumite, thus split, be rubbed down perpendicu- 
larly to the cleavage plane, no scales of mica are perceived 
be this new surface, because the observer is looking at the 
ges of the micaceous plates, but the lamellar nature of the | 
rock is thus made very apparent. a 
The chief constituent of the rock under the micréscope is _ 
quartz in very sharply fractured and fine grains, together with —_ 
a little mica. Occasionally are to be seen very minute black | 
grains. One specimen when rubbed down shows small pieces | 
of compact white quartz, giving it the appearance of a section 
of sausage. The specific gravity of the North Carolina speci- 
men, taken in the ordinary manner, is 2°61; but 2°69 after freeing 
its pores from air by a care gr. of quartz 2°65]. The — 
resuits : 
ee ee ee eee eae Pe 
¢ S beheh $3 pili 5 i . BS 
7 
a as 
analysis yields the following 
Vol, at red heat (water), ieee ae 0°17 
Silica, - <= - - - - 95°89 
tons opticn, oR ne ee Br 2°78 
Fe es Sa i eee ie 
+ Besides which, the uslistiie wiiguia yield 
NaO, MnO, MgO; bur no Al,0,. The | 
