58 
ECONOMIC GEOLOGY 
are isolated in schist or gneiss, and must have been formed 
by its recrystallization. As the mica is associated with tour- 
maline, 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 The original rock has been replaced 
by huge crystals of quartz, felspar (usually microcline), 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 iso- 
lated 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 un- 
conformably by limestone ; subsequently superheated acid 
water containing magnesia rose through the pyroxenite and