weed]  THE    GRIGGSTOWN,   N.   J.,    COPPER    DEPOSIT.  189 
also  carries  considerable  micaceous  hematite.  These  ore  minerals 
occur  in  the  leached  and  white  altered  horn  stone. 
The  ore  seam  appears  to  be  a  bedding  plane  of  the  old  shales,  along 
which  slipping  has  occurred  during  the  tilting  of  the  beds.  It  is,  so 
far  as  observed,  entirely  conformable  to  the  shales,  and  dips  west  at 
10°.  The  trap  sheet  exposed  on  the  summit  of  the  ridge  passes  under- 
neath the  coppper  bed,  and  the  exact  thickness  of  rock  between  the 
ore  and  trap  was  not  determined.  The  ore  seam  does  not  show  in  out- 
crop, and  the  mine  dumps  are  in  a  cultivated  field,  no  rock  appearing 
in  place.  However,  the  inclined  shaft  exposes  an  excellent  section  of 
the  altered  shales  overlying  the  ore,  and  the  ravine  scoring  the  slope, 
from  which  the  long  drain  tunnel  was  driven  nearly  a  century  ago, 
also  shows  good  exposures  of  the  shales. 
The  ore  seam  consists  largely  of  altered,  leached,  and  whitened  rock, 
with  patches  of  ore,  and  of  a  soft,  blackish  material  which  proves  to 
be  chlorite.  Cracks  run  down  and  connect  with  chlorite  bunches  in 
the  rock  below  the  ore  seam  proper,  but  no  copper  ore  was  found 
except  in  the  thin  seam  mentioned.  Samples  of  the  soft  chlorite,  which 
at  times  forms  a  layer  several  inches  thick,  showed  no  visible  copper 
minerals,  and  upon  assay  yielded  0.69  per  cent  of  copper.  The  ore 
follows  fractures  crossing  the  ore  seam  proper  and  is  not  uniformly 
distributed.  The  ore  body  worked  consists  of  bunches  of  glance  and 
oxidized  ores,  which  cover  irregularly  elliptical  areas  of  possibly  a 
couple  of  hundred  feet  across.  In  mining  so  thin  a  seam  the  underly- 
ing waste  has  also  been  broken  down.  There  is  said  to  be  a  vertical 
vein  of  copper  ore  exposed  in  this  shaft,  with  a  lower  layer,  a  sheet  of 
ore  on  the  150-foot  level,  but  no  evidence  of  this  could  be  obtained  on 
account  of  the  water.  Several  nearly  vertical  fractures  intersect  the 
ore  seam  near  the  incline.  These  fissures  are  filled  by  crushed  and 
altered  rock  and  carbonates,  and  are  said  to  be  gold  bearing,  but  assays 
made  in  the  Survey  laboratory  failed  to  show  even  a  trace  of  gold. 
The  scientific  interest  of  this  deposit  is  very  great,  on  account  of 
the  evident  reducing  action  of  the  hornblende  and  chlorite  upon 
copper-bearing  solutions,  but  the  discussion  of  the  origin  of  the  ore 
involves  a  consideration  of  the  various  cycles  of  uplift  and  erosion  to 
which  the  region  has  been  subjected  since  Triassic  time  and  the  accom- 
panying movements  of  percolating  waters,  which  are  supposed  to  have 
derived  the  copper  from  the  alteration  of  the  trap  from  a  fresh  basaltic 
lava  or  diabase  sheet  to  its  present  somewhat  altered  condition. 
Such  discussion,  together  with  an  account  of  the  impregnation  of 
the  white  sandstones  overlying  similar  intrusive  sheets  of  trap  rock  at 
Arlington,  New  Brunswick,  and  many  other  localities  in  New  Jersey, 
is  reserved  for  a  future  paper. 
