482  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1905. 
larger  masses  of  quart/  and  throughout  the  finer-grained  matrix.  No  graphite  was  noted 
in  the  larger  crystals  of  feldspar.  As  seen  in  the  thin  sect  ion  of  the  pegmatite,  the  graphite 
forms  irregularly  shaped  masses,  interlocking  with  the  quartz  grains.  These  flakes  and 
smaller  masses  range  from  1  to  one-half  millimeter  in  their  longer  diameters. 
A  chemical  determination  of  the  graphite  in  a  representative  sample  of  this  graphitic 
pegmatite  from  Yarmouth  was  made  by  Mr.  Sullivan  and  shows  «)  per  cent  to  be  present. 
ORIGIN    OF    THE   GRAPHITE. 
The  held  observations  and  microscopic  study  indicate  clearly  a  difference  in  the  char- 
acter of  the  graphite  in  the  two  occurrences  described.  The  Madrid  graphite  is  exceedingly 
fine  grained  and  of  the  variety  that  is  often  called  amorphous  graphite,  although  the 
minute  particles  are  in  reality  crystalline,  possessing  all  t  he  luster  of  the  larger  (lakes  of 
the  Yarmouth  graphite.  This  lack  of  similarity  in  size  of  particles  and  the  differences  in 
form  of  occurrence  are  suggestive  if  not  indicative  of  wholly  different  modes  of  origin. 
The  presence  of  graphite  in  the  Madrid  locality,  at  the  contact  between  an  intrusive  mass 
of  pegmatite  and  somewhat  carbonaceous  schist,  at  once  suggests  a  contact  origin  for  the 
graphite.  The  relative  concent  ration  of  the  graphite  at  the  contact  indicates  that  the 
processes  active  in  the  formation  of  that  mineral  were  connected  with  the  intrusion  of  the 
granitic  magma  rather  than  with  the  dynamic  forces  which  have  affected  to  some  extent  the 
rocks  of  the  region.  The  source  of  the  carbon  thus  crystallized  into  graphite  was  presumably 
in  the  original  sediments  from  which  the  beds  of  schist  were  formed.  This  inference  is  based 
on  tlu1  variation  in  content  of  graphite  in  adjoining  beds  even  at  the  pegmatite  contact,  a 
variation  believed  to  express  the  original  difference  in  percentage  of  carbon  in  the  successive 
layers  of  muddy  sediments.  The  possibility  is  recognized,  however,  that  certain  parts  of 
the  schist  may  have  exercised  a  selective  influence  in  the  concentration  of  the  graphite,  just- 
as  it  is  noted  that  these  adjoining  beds  contain  varying  amounts  of  tourmaline,  which  doubt- 
less originated  from  the  pegmatite  magma.  The  graphite  at  Madrid  is  believed  to  he  the 
product  of  the  conversion  and  concentration  of  carbonaceous  part  ides  of  sedimentary  origin 
through  the  agency  of  the  heated  vapors  issuing  from  the  intrusive  rock  magma  now  con- 
solidated as  pegmatite. 
In  the  Yarmouth  occurrence  there  is  no  evidence  of  any  source  of  the  carbon  of  the 
graphite  other  than  in  the  molten  rock  itself,  which  intruded  the  granite.  The  graphite  is 
as  much  an  essential  and  original  constituent  of  the  pegmatite  dike  as  is  the  quartz  or  the 
feldspar.  The  graphite  crystallized  possibly  later  than  the  feldspar-,  hut  plainly  earlier  than 
flu'  quartz,  and  like  these  minerals  was  of  magmatic  origin. 
ECONOMIC  CONSIDERATIONS. 
The  practical  questions  concerning  these  deposits  refer  to  the  quality  of  the  graphite  and 
the  quantity  present.  Certain  deductions  may  be  made  from  the  ohservations  given  above 
and  may  serve  to  answer  the  question  of  economic  worth. 
The  graphite  at  Madrid  is  of  the  type  often  called  amorphous,  a  relatively  low-priced 
material  except  when  fairly  pure  and  free  from  other  minerals.  As  has  been  noted  above, 
the  size  of  grain  in  this  graphite  is  minute  and  its  association  with  muscovite  and  quartz 
very  intimate.  These  two  characters  would  render  even  partial  separation  of  the  graphite 
and  mica  especially  difficult.  The  size  of  the  larger'  grains  being  only  0.2  mm.,  and  the 
average  being  less  than  0.04  mm.,  the  graphitic  rock  would  need  to  be  crushed  so  as  to  pass 
at  least  a  200-mesh  screen  before  the  complete  separation  of  graphite  from  the  mica  could 
he  effected.  Such  a  procedure  would  be  impracticable,  and  it  is  very  doubtful  if  crushing 
to  60-mesh  size  would  facilitate  separation  of  the  graphite  from  the  other  minerals.  In  short, 
t  he  intermixture  of  graphite  and  mica  is  on  so  minute  a  scale  as  to  prevent  recovery  of  any- 
thing hut  a  very  impure  graphite  by  any  known  method  of  separation. 
The  quantity  of  graphite  present  is  also  disappointing.  The  results  of  close  observation 
of  the  occurrence  at  Madrid  clearly  point  to  the  localization  of  the  graphite  at  the  contact 
