386          CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1906,  PART    T. 
a  relatively  small  percentage  of  the  total  mass  of  pegmatite  material. 
In   most   of  the   pegmatites  worked  commercially  the  feldspar  and 
quartz  crystals  do  not  average  more  than  4  or  5  feet  in  diameter. 
The  most  striking  characteristic  of  the  textun4  of  the  pegmatites  is 
its  extreme  irregularity.  In  a  mass  of  typical  granite  there  is  con- 
siderable uniformity  in  size  among  grains  of  the  same  mineral  species, 
hut  in  the  pegmatites  there  is  no  such  regularity.  A  feldspar  crystal, 
for  example,  is  as  likely  to  be  two  or  three  or  even  ten  times  as  large 
as  an  adjacent  costal  as  to  be  of  similar  size.  In  most  of  the  peg- 
matites there  is  also  much  graphic  granite,  consisting  of  an  intimate 
intergrowth  or  interpenetration  of  single  crystals  of  quartz  and 
feldspar,  the  quartz  forming,  on  certain  faces  of  the  feldspar  crystals, 
a  peculiar  angular  pattern  somewhat  resembling  the  cuneiform 
inscriptions  of  the  ancients.  Fine-grained  phases  pass  into  coarser 
graphic  granite  and  this  by  decrease  in  quartz  may  pass  into  masses 
of  pure  feldspar,  or  by  decrease  in  feldspar  into  masses  of  pure 
quartz.  Much  of  the  material  mined  as  "spar''  is  coarse-grained 
graphic  granite  containing  from  10  to  20  per  cent  of  free  quartz. 
In  the  great  majority  of  pegmatite  bodies  there  is  no  regularity 
whatsoever  in  the  distribution  of  the  different  minerals.  The  mica 
plates  show,  in  many  places,  a  tendency  to  group  themselves  along 
certain  planes,  but  these  seem  to  have  no  definite  orientation  with 
respect  to  the  general  outline  of  the  mass,  and  this  arrangement  is  by 
no  means  universal.  A  pegmatite  which  is  of  excellent  commercial 
quality  as  regards  its  feldspar  content  may  grade  within  a  short 
disi  ance  and  in  a  wholly  irregular  manner-  into  rock  which  is  worthless 
because  of  its  large  percentage  of  quart  z  or  t  he  abundance  of  biotite, 
black  tourmaline,  or  garnet. 
USES. 
FELDSPAR. 
Practically  all  of  the  feldspar  mined  in  Maine  is  used  in  pottery 
manufacture,  its  main  application  being  as  a  constituent  of  both 
body  and  glaze  in  true  porcelain,  white  ware,  and  vitrified  sanitary 
ware  and  of  the  "slip"  ( underglaze)  and  glaze  in  so-called  "porce- 
lain" sanitary  ware  and  enameled  brick.  The  amount  of  feldspar 
in  the  body  of  these  wares  generally  falls  between  15  and  35  percent, 
though  in  some  it  is  less  and  in  some  more.  In  glazes  the  percent- 
age of  feldspar  is  as  a  rule  between  30  and  50.  Small  amounts  of 
very  pure  spar,  carefully  hand  picked,  are  occasionally  shipped  for 
use  in  the  manufacture  of  artificial  teeth.  Feldspar  mined  in  other 
Eastern  States  is  also  used  principally  for  pottery  purposes,  though 
finding  some  application  in  glass  making,  in  dentistry,  and  in  the 
manufacture  of  certain  polishes  and  scouring  soaps  that  are  less 
abrasive  than  those  in  which  ground  quartz  is  used.      Much  interest 
