MICA    DEPOSITS    OF    WESTERN    NORTH    CAROLINA.  419 
first  formed,  followed  by  the  quartz,  and  then  in  the  middle  by  the 
streak  composed  chiefly  of  feldspar.  It  is  possible  that  the  feldspar 
formed  in  a  fissure  in  a  once  solid  mass  of  quartz;  though  it  probably 
occupies  the  portion  of  the  original  fissure  that  was  never  quite  closed 
by  the  quartz.  Fig.  17  represents  an  occurrence  that  is  uncommon, 
though  it  possesses  certain  features  bearing  on  the  origin  of  pegmatite. 
The  history  shown  by  this  deposit  may  be  briefly  described  as  follows: 
After  movements  associated  with  mountain  building  had  ceased — that 
is,  in  Carboniferous  time  or  later— the  granite  dike  was  forced  into  a 
fissure  or  line  of  weakness  cutting  across  the  foliation  of  the  country 
rock.  Cooling  was  fairly  rapid,  and  the  texture  was  consequently 
rather  fine.  This  was  followed  by  the  deposition  of  pegmatite  from 
solutions  passing  along  each  side  of  the  dike.  A  source  for  such  solu- 
tions could  be  found  in  the  final  stages  of  activity  of  the  magma  from 
which  the  granite  was  formed. 
These  two  illustrations  are  given  to  show  that  the  formation  of 
certain  pegmatites  is  much  more  readily  explained  by  referring  it  to 
aqueous  agencies  than  to  intrusion  as  an  igneous  body.  With  the 
probability  thus  established  of  an  aqueous  origin  for  certain  pegma- 
tites and  an  "intrusive  origin  for  others,  we  are  confronted  by  a  large 
number  the  nature  of  whose  origin  is  difficult  to  decide.  Among  these 
are  many  pegmatites  like  that  represented  in  fig.  15,  in  which  there  is 
a  banding  consisting  of  a  streak  of  feldspar  with  a  little  quartz  mixed 
through  it  lying  on  each  side  of  a  quartz  band.  Locally  there  are  sev- 
eral quartz  bands  in  the  pegmatite.  It  is  the  opinion  of  the  writer 
that  such  pegmatites  are  in  part,  if  not  wholly,  of  vein  origin.  The 
feldspar  component,  in  many  places  containing  more  or  less  quartz 
mixed  through  it,  may  have  been  intruded  as  a  magma  and  the  whole 
modified  by  the  secondary  introduction  of  quartz  by  solutions.  It 
seems  more  likely,  however,  that  if  a  part  is  to  be  considered  the  result 
of  aqueous  agencies  the  whole  pegmatite  should  be  so  considered.  A 
strong  argument  in  favor  of  the  deposition  of  these  quartz  bands  from 
solution  is  afforded  by  the  peculiar  banding,  apparently  not  due  to 
strain,  observed  in  the  quartz  streak  at  the  LyieCut  mine.    (See  p.  412.) 
The  blending  between  the  conditions  of  fusion  and  solution  con- 
ceived to  exist  under  heat  and  pressure,  such  as  prevail  in  deeply 
buried  granite  magmas  where  water  is  present  in  considerable  quan- 
tity, has  been  well  set  forth  by  Van  Hise,a  Crosby  and  Fuller,6  and 
Williams. c  As  stated  by  Van  Hise,  given  the  two  conditions,  a  magma 
and  a  solution  with  no  sharp  line  of  demarcation  between  them,  we 
may  expect  to  have  injections  of  dikes  and  aqueous  cementation, 
which  grade  into  each  other  and  between  which  no  sharp  distinction 
a  Van  Hise,  C.  R.,  A  treatise  on  metamorphism:  Mori.  U.  S.  Geol.  Survey,  vol.  47,  1904. 
f>  Crosby,  W.  O.,  and  Fuller,  M.  L  ,  Origin  of  pegmatite:  Am.  Geologist,  vol.  19,  1897. 
c  Williams,  G.  H..  General  relations  of  the  granitic  rocks  in  the  Middle  Atlantic  Piedmont  Plateau: 
Fifteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  1895. 
