!()()  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  L906,    PART    I. 
ID  such  places  no  sharp  line  can  be  drawn  between  the  point  where  the 
pegmatite  ceases  and  original  horse  begins. 
Pegmatite  is  closely  allied  to  granite  in  composit  ion.  As  in  granite, 
the  essential  constituents  are  feldspar  and  quartz,  with  more  or  less 
mica  and  other  accessory  minerals.  Though  hornblende  is  a  rather 
common  mineral  in  granite,  it  is  less  so  in  pegmatite.  Orthoclase  or 
microcline  are  the  most  common  varieties  of  feldspar  found  in  pegma- 
tite. In  many  places,  however,  a  variety  of  plagioclase,  either  albite 
or  oligoclase,  makes  up  part  or  all  of  the  feldspar  component.  The 
feldspar  occurs  in  masses  and  rough  crystals  with  a  diameter  of  sev- 
eral feet.  In  the  old  Mart  Wiseman  mine  near  Sprucepine,  Mitchell 
County,  orthoclase  crystals  '2  by  1  feet  and  larger  have  been  cut 
through.  From  their  whole  rectangular  cross  section,  as  exposed  in 
the  walls  of  the  tunnel,  a  simultaneous  reflection  of  light  is  obtained 
from  the  cleavage  faces. 
Quartz  assumes  various  forms  and  positions  in  the  pegmatite.  In 
many  places  it  bears  much  the  same  relation  to  the  feldspar  and  mica 
as  in  granite,  the  three  minerals  being  thoroughly  mixed  with  one 
another;  but  the  individual  grains  are  many  times  larger  than  in 
ordinary  granite.  Not  uncommonly  the  quartz  and  feldspar  assume 
a  graphic-granite  texture  in  n  portion  of  the  pegmatite.  Another 
common  feature  is  the  occurrence  of  large  separate  masses  of  quartz 
occupying  various  positions  in  the  pegmatite.  Such  quartz  masses 
may  be  irregular  in  form  and  but  little  influenced  by  t  he  shape  of  the 
pegmatite  or  inclosing  walls.  ( Generally ,  however,  t  hey  occur  in  bands 
or  sheets  lying  parallel  to  the  walls.  There  maybe  one  or  more  of 
these  quartz  bands  constituting  varying  proportions  of  t  he  pegmatite. 
Their  thickness  ranges  from  a  fraction  of  an  inch  \\y  to  dor  more 
feet.  Many  of  them  are  lenticular  in  shape,  the  length  varying  from 
four  <»r  live  to  twenty  or  more  times  the  thickness.  In  numerous 
places  these  quartz  streaks  or  veins  are  persistent  through  the  whole 
length  of  the  pegmatite  exposed.  Some  inclose  feldspar  or  mica 
bodies;  others  do  not.  The  quartz  of  these  segregations  is  massive 
and  generally  granular,  though  locally  crystallized.  In  the  latter 
case  it  may  he  1  ranslucent  or  clear  and  of  a  dark  smoky  or  light  color. 
It  is  generally  rat  her  pure  and  does  not  contain  feldspar  or  mica  in  any 
quantity. 
Muscovite  is  the  common  mica  of  pegmatite  and  is  the  only  variety 
mined  in  North  Carolina.  Biotite  occurs  in  some  quantity  in  a  few- 
deposits  and  in  smaller  amount  in  many  others.  Where  muscovite 
and  biotite  occur  together  in  a  deposit,  the  muscovite  is  generally 
clear  and  of  good  color.  It  is  not  unusual  for  the  two  to  occur  in 
intergrown  crystals  with  a  common  cleavage  plane.  Again,  mica 
from  deposits  in  rock  formations  where  the  ferromagnesian  minerals 
are  abundant,  as  hornblende  or  biotite  gneiss  and  schist,  is  generally 
