MICA   IN  THE   HARTVILLE   UPLIFT,   WYOMING. 
By  Sydney  H.  Ball. 
INTRODUCTION. 
During  the  field  season  of  1 906  the  writer  examined  the  mica  pros- 
jpects  of  the  Hartville  uplift,  situated  in  the  rugged  hills  among  which 
I Haystack  Peak  is  the  most  prominent.  (See  fig.  5,  p.  192.)  They  are 
J  located  upon  pegmatite  dikes  cutting  pre-Cambrian  schists.  These 
schists,  which  form  the  uppermost  member  of  the  older  sedimentary 
series, a  are  closely  folded  and  intensely  metamorphosed  near  the 
1  granite  of  the  Haystack  Hills.  The  mica-bearing  area  lies  to  the  east 
of  Whalen  Canyon  and  to  the  west  of  Cottonwood  Canyon,  and  its 
north  and  south  boundaries  are  respectively  McCanns  Pass  and  an 
east-west  line  passing  through  Haystack  Peak.  The  possibly  pro- 
ductive area  includes  all  or  parts  of  sees.  25,  26,  27,  34,  35,  and  36, 
T.  28  N.,  R.  65  W.,  and  sees.  1,  2,  and  3,  T.  27  N.,  P.  65  W. 
The  first  mica  claim,  named  the  Savage,  was  located  by  Joseph  L.' 
Stein  in  1881.  Other  prospects  were  soon  taken  up,  and  work  has 
been  done  on  some  of  them  up  to  the  present  day,  but  with  an  unim- 
portant exception  no  muscovite  has  been  shipped.  At  present  four 
prospects  appear  promising. 
Muscovite  in  commercial  quantities  is  confined  to  dikes  of  pegma- 
tite, which  cut  the  schists.  These  dikes  were  intruded  after  the 
schist  became  fissile,  and  in  consequence  trend  in  the  main  parallel  to 
the  schistosity,  although  in  places  they  cut  across  it.  The  dikes  vary 
from  thin  stringers  to  irregular  intrusive  masses  one-fourth  mile  wide. 
In  many  places  their  width  is  constant,  but  here  and  there  it  changes 
markedly  within  a  short  distance.  The  pegmatite  is  composed  of  the 
following  minerals,  named  in  the  order  of  their  abundance :  Feldspar 
(orthoclase,  microcline,  and  albite),  quartz,  muscovite  (white  mica), 
black  tourmaline,  beryl,  brown  garnet,  and  biotite.  It  is  in  general 
coarsely  granular,  the  mineral  bodies  varying  in  diameter  from  one- 
fourth  inch  or  less  to  2  feet  or  more.  Such  extreme  variations  are 
rarely  seen  in  a  single  dike,  although  important  changes  in  the  sizes  of 
grains  take  place  within  comparatively  short  distances.  '  The  distribu- 
tion of  the  minerals  in  the  pegmatite  is,  as  a  rule, fairly  even,  although 
a  For  the  succession  of  the  pre-Cambrian  rocks  of  the  Hartville  uplift  see  pp.  193-194  of  this  volume. 
423 
