388         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  L906,  PART   1. 
diameter.  A  single  load  of  feldspar  is  usually  ground  for  four  to  six 
hours  in  these  mills  and  in  that  time  is  reduced  to  a  fineness  of  at 
least  200  mesh.  The  material  is  then  ready  for  shipment  either  in 
bulk  or  in  bags. 
Quartz  used  for  pottery  purposes  is  ground  in  the  same  manner 
as  feldspar,  but  none  is  now  being  produced  in  Maine. 
COMMERCIAL  AVAILABILITY   OF   DEPOSITS. 
The  answer  to  the  question  whether  it  will  pay  to  work  a  given 
feldspar  or  quartz  deposit  i^>  dependent  on  a  number  of  different  fac- 
tors. Considered  as  a  whole,  the  Maine  deposits  have  the  disadvan- 
tage, as  compared  with  those  <^l'  Connecticut,  New  York,  Pennsylvania, 
and  Maryland,  of  being  far  from  the  markets,  a  feature  which  renders 
the  mining  of  quartz  wholly  unprofitable.  The  hulk  of  the  material 
mined  in  Maine  under  the  commercial  name  of  "spar"  is  not  pure  feld- 
spar, hut  an  association  of  feldspar  and  free  quartz,  usually  intergrown 
in  the  form  of  a  coarse  graphic  granite.  In  the  past  a  number  of  the 
quarries  have  produced  much  larger  amounts  of  feldspar  free  from 
quartz  than  can  now  he  mined.  The  requirements  of  the  potter's 
trade  demand  that  iii  genera]  the  percentage  of  free  quartz  associated 
with  the  feldspar  in  the  ground  product  shall  not  exceed  L5  or  20  per 
cent,  and  certain  potters  demand  a  spar  which  is  nearly  pure,  con- 
taining  probably  less  than  5  percent  of  free  quartz.  In  order  to  be 
profitably  worked,  in  most  feldspar  mines  in  Maine,  between  one-fourth 
and  one-half  of  the  total  material  quarried  must  carry  under  15  to  20 
per  cent  of  free  quart  /.. 
A  factor  of  the  utmost  importance  is  the  amount  and  distribution 
of  the  iron-bearing  minerals  black  mica,  garnet,  and  black  tourma- 
line. For  pottery  manufacture  the  spar  must  he  practically  U-vc  from 
these  minerals,  which  if  present  in  the  ground  spar  produce  hrown 
discolorations  in  white  ware  on  burning.  To  he  workable  commerci- 
ally, these  minerals  musl  he  so  rareorso  segregated  in  certain  portions 
of  the  deposit  thai  they  can  he  separated  from  the  feldspar  without 
much  more  hand  sorting  and  cobbling  than  is  necessary  anyway  in  the 
separation  of  the  highly  feldspathic  material  from  that  which  is  highly 
quartzose.  A  number  of  coarse-grained  masses  of  pegmatite  with 
feldspar  of  excellent  quality  are  rendered  worthless  for  pottery  uses 
by  the  ahundance  of  one  or  more  of  these  iron-hearing  minerals.  The 
presence  here  and  there  of  minute  flakes  of  white  mica  (muscovite) 
can  hardly  be  avoided  even  in  the  highest  grades  of  commercial  feld- 
spar, and  chemically  this  mineral  is  not  injurious.  It  is,  however, 
exceedingly  difficult  to  pulverize  the  flakelike  flexible  plates  of  mica 
to  a  fineness  equal  to  that  attained  \)\  the  feldspar,  and  it  is  therefore 
necessary  in  mining  to  separate  the  muscovite  carefully  from  the  spar. 
