422  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PABT    I. 
other  pegmatite  masses,  room  for  the  segregation  was  probably  pro- 
duced by  the  expansive  force  of  the  growing  feldspar  crystals,  which 
have  crowded  the  gneiss  or  schist  out  on  each  side. 
SUMMARY   OF    CONCLUSIONS. 
It  is  uncertain  whether  pegmatites  should  be  called  intrusions  or 
vein  formations.  It  is  probable  that  some  are  dikes  and  others  are 
veins.  Those  with  a  typical  coarse  granitic  texture  are  probably  of 
intrusive  origin;  those  with  a  banded  structure  are  probably  the 
result  of  aqueous  deposition.  In  view  of  certain  examples,  already 
illustrated  by  figures,  these  statements  become  more  acceptable. 
Van  Hise's  conception  of  the  condition  of  pegmatite  material  before 
the  pegmatite  was  formed  seems  particularly  applicable  in  this  region; 
that  is,  given  a  magma  and  a  solution  with  no  sharp  line  of  demarca- 
tion between  them  there  may  be  intrusions  and  aqueous  cementation 
also  grading  into  each  other.  Graton's  interpretation  of  the  forma- 
tion of  the  auriferous  quartz  veins  of  the  southern  Appalachians  also 
seems  particularly  appropriate  in  accounting  for  pegmatite  veins; 
that  is,  the  solutions  were  forced  into  fractures  or  fissures  under  great 
pressure  and  by  the  aid  of  the  expansion  produced  by  the  crystalliza- 
tion of  the  minerals  being  deposited,  spread  the  walls  apart  sufficiently 
to  allow  the  formation  of  the  veins.  The  occurrence  of  much  peg- 
matite in  small  streaks  through  the  rock  formations  is  probably 
caused  either  by  recrystallization  through  the  combined  action  of 
water  and  heat  or  by  solutions  being  forced  through  fractures  or 
seams  and  depositing  their  loads,  or  by  both.  It  might  also  be  pos- 
sible for  Mich  pegmatization  to  be  produced  \>\  the  injection  of  an 
extremely  fluid  aqueo-igneous  magma  into  and  through  the  forma- 
tions. Disconnected  bodies  of  pegmatite  are  also  more  readily 
explained  by  deposition  from  solution  than  by  intrusion  as  dikes. 
The  question  of  the  origin  of  pegmatites  is  chiefly  of  scientific 
interest  rather  than  of  commercial  importance;  for  good  deposits  of 
mica  arc  found  in  rock--  of  both  dike  and  vein  types.  The  quality  of 
the  mica  from  one  type  is  in  general  no  better  than  that  from  t lie 
other.  It  is  probable,  however,  that  those  pegmatites  which  are 
typically  of  intrusive  origin  will  In'  found  to  hold  out  longer  than 
those  with  veinlike  structure.  Although  the  available  evidence  is 
insufficient  t<>  prove  this  definitely,  there  are  certain  dikelike  masses 
that  have  been  followed  long  distances  or  to  considerable  depth  and 
found  to  carry  paying  mica  to  the  limits  worked.  On  the  other  hand, 
many  veinlike  deposits  have  been  opened  and  large  (plant  it  ies  of  mica 
recovered  from  certain  portions,  and  then  the  "vein"  has  abrupt ly 
become  poorer  or  pinched  out.  Of  course  there  are  veinlike  deposits 
which  have  been  worked  through  considerable  distances;  though 
probably  none  have  held  out  so  persistently  as  the  hodies  of  intrusive 
origin. 
