408  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    T. 
smaller  biotite-gneiss  bodies  included  in  the  large  area  of  hornblende 
gneiss  which  forms  the  country  rock  of  the  region.  The  rocks  as 
exposed  in  the  mine  are  approximately  horizontal,  with  only  a  few 
gentle  monoclinal  folds  dipping  southward. 
A  tunnel  has  been  driven  in  oyer  450  feet  in  a  southerly  direction 
From  this  drifts  have  been  run  off  both  to  the  east  and  the  west  for 
distances  varying  from  25  to  60  feet.  The  main  tunnel  was  carried 
back  by  a  series  of  rooms,  some  being  about  25  feet  wide,  where  the 
"vein"  was  found  sufficiently  rich.  The  size  and  shape  of  these 
rooms  depended  on  the  pegmatite  lens  thai  was  removed  for  mien  as 
they  were  being  made.  In  some  places  all  of  the  pegmatite  had  been 
remoA'ed  on  one  side  of  the  room  or  the  other.  In  others,  however,  a 
streak  a  few  inches  thick  was  left  in  the  walls,  showing  where  the 
lens  had  pinched  down  from  several  feel  (the  height  o\'  the  room)  to 
a  few  inches.  The  structure  of  the  pegmatite  is  strikingly  illustrated 
in  the  walls  and  faces  of  the  workings,  where  cross  sections  of  the 
lenses  can  be  seen.  Some  are  (>  to  H)  inches  thick  and  a  couple  of 
feet  long,  others  are  of  much  greater  thickness  (up  to  5  or  t>  feel )  and 
of  proportional  or  even  greater  length.  These  lenses  overlap  or  lie 
parallel  to  each  other.  Many  of  them  lie  it)  the  same  strata  of  the 
gneiss,  though  separated  by  several  feet.  A  thin  seam  or  parting, 
locally  containing  a  little  pegmatite  material,  can  generally  be  traced 
between  two  such  lenses.  Here  and  there  the  pegmatite  occurs  in 
sheets  or  streaks,  which  in  places  bulge  oul  into  lenticular  form. 
These  streaks  may  pinch  down  to  mere  threads,  but  when  followed  a 
little  way  open  out  into  other  lenses.  The  full  thickness  of  the  belt 
of  overlapping  and  parallel  pegmatite  lenses  and  streaks  is  generally 
under  8  feet.  The  mica  gneiss  curves  around  swells  and  bulky  parts 
of  the  lenses. 
From  lli«-  mouth  of  the  tunnel  the  pegmatite  outcrop  has  been 
traced  both  t<>  the  east  and  the  west  for  some  distance  around  the 
contour  of  the  mountain  side.  A  diabase  dike,  called  "the  iron  bar" 
by  the  miners,  follows  the  pegmatite  back  as  faras  development  work 
lias  been  carried.  It  is  very  irregular,  appearing  in  one  place  cutting 
into  the  pegmatite  and  then  not  seen  again  for  some  distance. 
The  texture  and  composition  of  the  pegmatite  are  those  of  very 
coarse  granite.  The  three  constituent  minerals  are  thoroughly 
mixed  and  have  separated  out  in  large  masses.  Even  in  lenses  only 
10  inches  thick  mica  crystals  5  or  6  inches  in  diameter  have  been 
found.  The  quality  of  the  mica  is  excellent.  The  color  is  a  clear- 
light  "rum''  when  the  sheets  are  about  one-eighth  inch  thick.  The 
lamination  is  perfect,  and  beautiful  sheets  for  glazing  purposes  can 
be  obtained.  The  yield  for  the  amount  of  rock  removed  is  satis- 
factory and  contains  a  fair  proportion  of  larger  sizes. 
