FELDSPAR    AND    QUARTZ    DEPOSITS    OF    MAINE.  385 
ORIGIN   AND   AGE. 
It  is  not  possible  here  to  enter  into  a  detailed  discussion  of  the 
origin  of  these  pegmatites.  There  can  be  no  doubt,  however,  that 
they  represent  simply  one  phase  of  the  granitic  intrusions  so  abun- 
dant in  the  southern  and  southeastern  parts  of  the  State.  The 
evidence  of  this  is  found  in  their  distribution  with  respect  to  the 
areas  of  true  granite,  in  the  presence  of  dikes  and  irregular  intrusions 
of  true  granite  in  all  the  regions  where  pegmatite  occurs,  and  in  an 
actual  gradation  from  pegmatite  into  fine-grained  granite  observed 
at  many  localities.  The  granites  of  the  eastern  part  of  Maine  are 
known  to  be  late  Silurian  or  Devonian  in  age,  and  there  is  every  reason 
to  believe  that  the  granites  of  southern  Maine,  with  their  associated 
pegmatites,  are  of  similar  age.  They  are  certainly  later  than  the 
period  of  dynamic  metamorphism  at  the  close  of  the  Ordovician. 
GENERAL    CHARACTERS. 
COMPOSITION. 
The  pegmatites  in  all  parts  of  the  State  show  great  similarity  in  the 
principal  minerals  developed,  although  they  exhibit  notable  differ- 
ences in  the  minor  constituents.  In  mineral  composition  they  are 
essentially  coarse  granites,  the  principal  light-colored  constituents 
being  potash  feldspar,  quartz,  and  muscovite,  and  the  principal 
dark-colored  constituents  black  mica  (biotite)  and  black  tourmaline. 
In  pegmatites  where  black  mica  is  abundant,  black  tourmaline  is,  as  a 
rule,  rare  or  absent,  and  vice  versa.  Accessory  constituents  that  are 
almost  invariably  present  are  garnet,  magnetite,  and  opaque  green 
beryl.  Accessory  minerals  that  are  present  only  in  certain  pegma- 
tites number  over  50  species;  but  perhaps  the  most  important  are 
lepidolite  or  lithium  mica;  blue,  green,  and  pink  tourmaline;  trans- 
parent green  or  golden  beryl,  topaz,  and  amethystine  quartz.  In 
some  places,  as  at  Mount  Mica,  in  Paris,  Oxford  County,  certain  of  the 
gem  minerals  are  present  in  considerable  quantity  and  are  of  the 
finest  quality,  so  that  the  pegmatite  can  be  profitably  exploited  as  a 
gem  deposit. 
COARSENESS    AND    TEXTURE. 
The  pegmatites  show  remarkable  variation  in  coarseness,  some, 
especially  the  narrower  dikes  and  sills,  being  little  coarser  than 
coarse-grained  granites,  though  differing  from  the  latter  in  texture. 
In  others  single  crystals  of  nearly  pure  feldspar  may  be  20  feet  across, 
and  single  beryl  crystals  may  reach  the  diameter  of  a  hogshead.  The 
major  part  of  the  pegmatites  are  nearer  the  lower  limit  of  coarseness 
than  the  higher.  Only  the  coarser  bodies  are  commercially  valuable 
for  their  feldspar,  quartz,  mica,  or  gem  minerals,  and  these  constitute 
