THE MICAS, ASBESTOS, AND GEMS 159 
formed a seam of coarse calcite containing books of phlogo- 
pite. In other cases the mica has been developed in dykes 
on the margin of pyroxenite. The age of the mica is pre- 
Cambrian as some lodes are cut off unconformably by the 
Cambrian rocks. 
Mining Ecowomics—Few mica fields can compete with 
India owing to the cheapness and efficiency of Indian labour. 
As the value of mica varies with the quality and size of the 
NW 
Fie. 49.—PaLocorITE LopE aT LouGHBOROUGH, CANADA, 
Pyroxenite (v) intrusive into gneiss (inclined lines) and overlain by an 
altered limestone (c). A lode (}) of coarsely crystalline calcite rises 
from the pyroxenite and includes and is bounded by mica books (short 
strokes). A smaller lode with mica occurs in the pyroxenite, (After 
Schmid.) 
cut pieces, the economics of mica mining do not depend on 
the weight of output. The mica is usually won by small 
Pits sunk along the margin of the pegmatite blows or veins, 
The mica is of little value where it is most abundant, for the 
books are interwoven and do not yield large leaves. The 
rough mica crystals are split into thin leaves, which are cut 
into the largest sizes possible after imperfections are removed 
and the edges trimmed. The pieces are graded into sizes, 
Those of No. 7, about 1 inch square, are used as washers in