C. M. Wetherill on Itacolumite. 65 
ns 
composing the groups averages between (0-026)? to (0°04)? mm. 
The on cubic capacity of a group or grain could be deter- 
min iplyi 
great uniformity with respect to size. 
e scales of mica are flat and nearly square fragments. 
They average in area (0:08)? mm. and vary from (0°26)? to 
(0065)? mm. 
The Brazil mineral presents almost exactly the same charac- 
ter under the microscope as that from North Carolina, with the 
exception that the individual grains, as well as the congeries, are 
somewhat larger in the former. The oxyd of iron in the min- 
is not the cement by which the grains are united, for a thin 
section of the rock boiled in HCl for five hours did not become 
more friable. The cement would appear rather to be siliceous 
ydrated silica); because a piece boiled in a solution of caustic 
soda becomes partially disintegrated, especially at the edges, and 
18 much more friable as to its mass. ; 
When dilute solution of hydrofluoric acid is poured upon a 
thin section it is immediately resolved into grains, affording a 
complete disintegration of the section. 
34mm. (12 in.) diameter, and may be made to touch every point 
in the area of the approximate spherical zone. e rod Iso 
be twisted about its axis, the torsion being 10°. 
ry.— When the rod is supported upon two nails 185 mm. 
apart, it assumes the form of a curve very like the ecatenary. 
The greatest deflection from the line joining the points of sup- 
‘Aue. Jour, Se1.—Szconp Suatms, Vou. XLIV, No. 190.—Juzr, 1867. 
9 
port Varies from 3 to 3} mm. as the rod is rotated about itsaxis 
