60 
ECONOMIC GEOLOGY 
electrical apparatus, and worth sometimes 7d. per Ib.; 
No. I contains pieces of 24 to 36 square inches and are worth, 
according to clearness, from 10s. to 30s. per Ib. ; extra special 
sizes may sell for 37s. per lb. The economics of mica mining 
may be illustrated by the results at Kodarma in Bihar. 
Each miner obtained an average of 32 lb. of rough mica per 
day; this yielded 4 Ib. of cut mica. If this had all been 
No. 7 mica at 7d. per lb., the yield would only have been 
2s. 4d. per man, but its average price before the War was 
0s. per 1b., and as wages were 6d. per day, the yield was 
profitable. 
ASBESTOS 
Asbestos (from a Greek word meaning inextinguishable) is 
a fibrous mineral which is non-inflammable, and a bad con- 
ductor of heat, and is produced by the decomposition of several 
magnesian silicates. Flexible fibres are woven .into fire- 
proof cloth ; others are used for making heat-proof tiles and 
packing, filters, and cement; the largest use at present is in 
motor car brakes and clutches. 
The most important variety is chrysotile asbestos, a 
fibrous serpentine worked chiefly in Canada, Rhodesia, the 
Ural Mountains, and Cyprus. The main asbestos field is 
at Thetford and Black Lake in Quebec. The asbestos occurs 
in veins in a thick sill of Devonian peridotite, diabase, and 
porphyry, which contains veins and masses of pegmatite, 
granite, and aplite. The best asbestos js said to occur 
abutting against the aplite, and asbestos tufts occur in its 
quartz. The serpentine is due to the alteration of peridotite, 
and contains masses and lenticular streaks of it. The ser- 
pentine is traversed, often along shearing planes, by veins of 
asbestos up to 3—6 inches thick. The fibres are at right- 
angles to the vein, and have formed by recrystallization 
of the serpentine along both sides of a fissure. A variety, 
locally called fibrolite, at the Vimy Ridge Quarry has longer 
but more brittle fibres. The asbestos has been worked in 
long open cuts, now 350 feet deep, and has been proved by 
boring down to 800 feet. The fibre is separated from the 
crushed rock by shaking tables and air suction, and amounts 
on an average for the district to 4 per cent. of the rock. 
Tremolite or Italian asbestos is worked in Piedmont. and has