58

ECONOMIC GEOLOGY

are isolated in schist or gneiss, and must have been formed
by its recrystallization. As the mica is associated with tourmaline,
 fluorite, topaz and china-clay, it must have been
formed pneumatolytically by super-heated steam containing
boric and fluoric acids. The original rock has been replaced
by huge crystals of quartz, felspar (usually microcline),
and “books” of mica. The quartz may form the core of
a mass or the middle of a sheet of pegmatite (Fig. 48). The
mica books usually occur at the contact of the pegmatite
with this quartz or with the country, especially where the
ountry rock is strongly micaceous.
Phlogopite, a brown magnesian mica, is of especial value
for electrical purposes because it is softer than muscovite

F16. 48—THE ForMaTION OF Mica IN
Gne1ss, Kenva CoLony.
{Gn.), A pegmatite vein (P) with isolated
 pegmatite bodies (P) in the
gneiss. The core of the pegmatite
Consists of quartz, Q. Muscovite
* books *’ especially abundant near
the quartz.

Cn

and rubs away at the same rate as copper. Bearings with
phlogopite insulators wear evenly. The chief field is in
Ottawa, Canada, where the gneiss and crystalline limestones
are intersected by veins or masses of pyroxenite. According
to some authorities (e.g. A. Osann, Ann. Rep. G.S. Canada,
Xii, 1002, p. 21) the pyroxenite is intrusive ; according to
F. D. Adams and Barlow (G.S. Canada, Mem. 6, 1910, pp.
88-90) it is altered limestone. In some cases, such as the
Loughboro’ Mine (Fig. 49 after Schmid, Mica, Canada Dept.
Mines, No. 118, 1912, P- 147), the pyroxenite would appear
intrusive into the gneiss; both rocks were covered unconformably
 by limestone ; subsequently superheated acid
water containing magnesia rose through the pyroxenite and